Types of SMS Services — Promotional, Transactional, OTP & Flash SMS Explained
- TechTo Networks
- Jun 9, 2025
- 27 min read
Updated: May 12
SMS is not one thing. It is a family of services — each with its own delivery rules, regulatory category, timing restrictions, sender ID requirements, DLT registration path, and ideal use cases. Using the wrong type costs you money, gets your messages blocked, and can result in TRAI penalties.
This guide covers every type of SMS service used in India — Promotional, Transactional, OTP, Flash, Missed Call, Unicode, Bulk, and API SMS — with the full DLT compliance rules, real message examples, timing windows, sender ID formats, pricing ranges, and a decision framework to help you pick the right service type for every use case.
Why Getting the SMS Type Right Matters
Most businesses treat SMS as a single channel. It is not. Under India's TRAI TCCPR 2018 regulations and the DLT (Distributed Ledger Technology) framework, every commercial SMS must be classified into one of four regulatory categories before it can be sent. The wrong classification leads to:
Messages blocked at the carrier level before reaching the recipient
Sender IDs blacklisted by TRAI
Financial penalties for non-compliant commercial messaging
Delivery failures during peak OTP and transaction windows
Customer complaints and brand damage
Understanding the distinct types of SMS services — what each one is, when to use it, what DLT category it falls under, and what rules govern it — is the foundation of any compliant, effective SMS strategy in India.
The Four DLT Regulatory Categories Under TRAI
Before breaking down service types, it helps to understand the four regulatory categories that TRAI uses to classify all commercial SMS in India. Every SMS service type maps to one of these categories on the DLT portal:
DLT Category | Common Name | Can Reach DND? | Time Restrictions | Sender ID |
Service Implicit | Transactional | Yes | None (24/7) | Alphanumeric (6 chars) |
Service Explicit | Opt-in Service | Yes (with consent) | None (24/7) | Alphanumeric (6 chars) |
Promotional | Promotional | No | 9 AM – 9 PM only | Numeric (6 digits) |
Transactional (Legacy) | OTP / Bank alerts | Yes | None (24/7) | Alphanumeric (6 chars) |
Each of these maps to the SMS service types described in detail below. Now let us go through every type.
1. Promotional SMS
What Is Promotional SMS?
Promotional SMS is any text message sent to customers or prospects with the intent to advertise, market, sell, or promote a product, service, offer, event, or brand. It is the "loudspeaker" of business messaging — designed to attract attention, generate clicks, and drive conversions.
Promotional SMS is the most commonly used and most tightly regulated SMS type in India. It falls under the Promotional DLT category and is governed by strict time windows and DND (Do Not Disturb) restrictions.
Key Characteristics of Promotional SMS
Who it reaches: Only non-DND numbers. TRAI's DND registry blocks promotional messages from reaching numbers that have opted out of commercial SMS. Your SMS provider must filter DND numbers from your list before dispatch.
When it can be sent: Between 9:00 AM and 9:00 PM only, seven days a week, including Sundays and public holidays. Promotional messages dispatched outside this window are blocked by carriers.
Sender ID format: A 6-digit numeric header (e.g., 567890). Unlike transactional messages, promotional SMS cannot use an alphanumeric brand name. The 6-digit number is assigned by the DLT portal after registration.
DLT registration: Required. You must register your promotional sender ID and every message template on the DLT platform (Vodafone Idea, Jio, Airtel, BSNL, TATA, etc.) before sending.
Cost: The most affordable SMS type — typically ₹0.10 to ₹0.18 per SMS depending on volume. Promotional routes are cost-optimized because they carry lower delivery guarantees and timing flexibility than transactional routes.
DLT Rules for Promotional SMS
Sender ID must be a 6-digit numeric header registered under the Promotional category.
Message template must be registered under the Promotional category on the DLT portal.
Template content must not contain any transactional language (account numbers, OTPs, order details) — mixing promotional and transactional content in one message is a violation.
Messages can only be sent to non-DND numbers. Sending to DND-registered numbers is a TRAI violation.
Delivery windows are strictly 9 AM to 9 PM. Any attempt to dispatch outside this window is rejected by the carrier at the network level.
Opt-out mechanism: Every promotional campaign should provide a clear way for recipients to opt out (e.g., "Reply STOP to unsubscribe" or a visible opt-out link).
Real Promotional SMS Examples
Retail / E-Commerce:
"BIG SALE! Get 40% off all orders above ₹999. Use code SAVE40. Shop now: www.store.in. Valid till midnight. -XYZ Store"
Restaurant / Food Delivery:
"🍕 Hungry? Get 30% off your next order from Pizza Palace. Offer valid today only. Order now: bit.ly/pp30. -PPIZZA"
Travel / Hospitality:
"Exclusive! Book a 3N/4D Goa package starting ₹7,999 per person. Limited seats. Call 1800-XXX-XXXX or visit www.travel.in -GOATR"
EdTech / Coaching:
"LAST CALL: Enroll in our JEE 2026 batch before June 30. Seats filling fast. 92% selection rate. Call now: 9999XXXXXX. -JEEAC"
Finance / Insurance:
"Renew your health policy before expiry. Get ₹500 cashback on renewal. Call 9898XXXXXX. T&C apply. -HLIFE"
When to Use Promotional SMS
Use promotional SMS when:
You are running a sale, discount, or limited-time offer.
You are announcing a new product launch, event, or service.
You are sending seasonal or festive campaigns (Diwali, Holi, year-end sale).
You are re-engaging dormant customers with an incentive.
You are sending loyalty rewards, birthday coupons, or referral offers.
Your recipient has not necessarily performed a prior transaction — you are initiating contact.
Do NOT use promotional SMS when:
You need to reach DND-registered numbers.
You need to send messages after 9 PM (e.g., midnight flash sale alerts).
The message contains OTPs, account information, or confirms a user-initiated transaction.
Promotional SMS Best Practices
Keep the message under 160 characters to avoid multi-segment billing. Include the offer, the brand, and a clear CTA within one segment.
Segment your contact list. Send clothing offers to customers who previously bought clothing, not to everyone.
Use URL shorteners with tracking (like bit.ly or your provider's link shortener) to measure clicks.
Never include misleading claims, false urgency (e.g., "You have won!"), or deceptive sender names.
Send at peak engagement times — research consistently shows 12 PM–2 PM and 6 PM–8 PM yield the highest open rates for promotional campaigns.
Cap frequency. More than 2-3 promotional SMS per week per contact leads to opt-outs and complaints.
2. Transactional SMS
What Is Transactional SMS?
Transactional SMS is any message that communicates essential, non-promotional information about a service, account, or transaction that the recipient has initiated or is already using. It delivers information the recipient expects and needs — not advertising.
Under DLT classification, transactional messages fall under Service Implicit (existing customer relationship, no explicit opt-in needed) or Service Explicit (requires prior opt-in consent). Both categories bypass DND and have no time restrictions.
Key Characteristics of Transactional SMS
Who it reaches: All numbers — including those registered on the DND list. Because transactional SMS delivers information tied to a user's own service activity, TRAI grants it DND bypass rights. This is the most important difference from promotional SMS.
When it can be sent: 24 hours a day, 7 days a week — no time restrictions whatsoever. An OTP sent at 3 AM, a banking alert triggered at midnight, or an emergency notification sent on a Sunday all go through without restriction.
Sender ID format: A 6-character alphanumeric header representing the brand (e.g., TECHTO, HDBANK, FLIPKR). This branded sender ID builds immediate trust — the recipient knows who sent the message before reading it.
DLT registration: Required. The sender ID and message templates must be registered under the Service Implicit or Service Explicit category on the DLT portal. Template variable fields must be marked with {#var#} placeholders.
Cost: Slightly higher than promotional — typically ₹0.15 to ₹0.22 per SMS — because transactional messages use premium, dedicated carrier routes with higher delivery guarantees and faster latency.
Service Implicit vs. Service Explicit — What Is the Difference?
This distinction confuses many businesses. Both are transactional, but:
Service Implicit — Messages where the relationship and service expectation is already established. The customer signed up, completed a transaction, or is actively using a service. No explicit opt-in for SMS is needed because the communication is an inherent part of the service. Examples: OTP after a customer clicks "Login," order confirmation after checkout, bank alert after a debit.
Service Explicit — Messages that require prior explicit consent because they go beyond the immediate transaction. Examples: Periodic account statements, subscription renewal reminders, health check-up reminders to registered patients. The customer must have opted in specifically to receive these communication types.
DLT Rules for Transactional SMS
Sender ID must be a 6-character alphanumeric header registered under Service Implicit or Service Explicit.
Every template must be approved by the DLT portal under the correct service category.
Template variable fields must use the {#var#} format. Dynamic content like names, order IDs, amounts, and OTPs are entered via API at send time and must match the registered variable structure.
The message body must not contain promotional language, offers, discounts, or marketing content. Adding "Use code SAVE10 for your next order" to an order confirmation immediately reclassifies it as promotional — making it a TRAI violation.
There is no restriction on DND numbers or sending time.
Message content must relate directly to a service or transaction initiated by or pertaining to the recipient.
Real Transactional SMS Examples
Banking / Finance:
"HDBANK: Rs.15,000 debited from your account XX1234 on 03-Jul. Available balance: Rs.42,330. Not you? Call 1800-XXX-XXXX. -HDBANK"
E-Commerce — Order Confirmation:
"Your order #ORD98721 for Nike Air Force (Size 9) has been confirmed. Expected delivery: 5-Jul. Track: bit.ly/trk98721. -FLIPKR"
Healthcare — Appointment Reminder:
"Reminder: Your appointment with Dr. Meera Sharma is on 04-Jul at 10:30 AM. Apollo Clinic, Bandra. Reply CNFM to confirm. -APOLLO"
Travel — Booking Confirmation:
"Booking confirmed! PNR: 8UX9K2. Mumbai→Delhi, 05-Jul 07:45. Web check-in opens 48hrs before departure. -AIRINB"
E-Commerce — Shipping Update:
"Great news! Your order #ORD98721 has been shipped via Delhivery (AWB: 12345678). Track real-time: bit.ly/dlv98721. -FLIPKR"
OTP / Authentication:
"Your OTP for login to MyBank is 847203. Valid for 5 minutes. Do not share this with anyone. -MYBNK"
When to Use Transactional SMS
Use transactional SMS when:
You are sending an OTP, verification code, or 2FA token.
You are confirming an order, booking, reservation, or payment.
You are sending shipping, delivery, or logistics status updates.
You are sending account balance alerts, debit/credit notifications, or bank statements.
You are sending appointment, meeting, or event reminders to users with existing bookings.
You need to reach DND-registered numbers.
You need to send at any time of day or night.
The message content is informational — not a sales pitch.
3. OTP SMS (One-Time Password SMS)
What Is OTP SMS?
OTP SMS is a specialized subset of transactional SMS purpose-built for delivering one-time passwords, verification codes, authentication tokens, and PIN codes. It is the fastest, highest-priority messaging category — because a 10-second delay on an OTP is the difference between a successful transaction and a customer abandoning the process.
While OTP is technically a subcategory of transactional SMS under DLT, it functions differently in practice. OTP messages use express carrier routes — dedicated pathways that bypass shared queues used for standard transactional and promotional traffic. Many SMS providers maintain separate OTP infrastructure precisely because of these latency requirements.
Key Characteristics of OTP SMS
Delivery speed requirement: Industry standard is delivery within 3 to 5 seconds of the API call. Anything over 10 seconds is considered a failure for most OTP use cases. For payment authentication and banking OTPs, NPCI (National Payments Corporation of India) guidelines require even faster delivery.
Who it reaches: All numbers including DND. OTP is classified under Service Implicit — the highest trust level on the DLT framework.
Time restrictions: None — OTPs can be sent 24/7.
Sender ID: 6-character alphanumeric header (same as transactional). For BFSI (Banking, Financial Services, Insurance) use cases, TRAI has specific guidelines on sender ID formats.
Cost: Slightly higher than standard transactional — typically ₹0.18 to ₹0.28 per SMS — due to premium express routing infrastructure.
Message validity: OTPs are time-bound. Always state the validity window in the message body ("Valid for 5 minutes," "Valid for 10 minutes"). TRAI guidelines and security best practices both require this.
OTP Delivery Architecture — How It Works
Standard transactional SMS flows through shared carrier routes. OTP SMS — on a quality platform like TechTo Networks — flows through a dedicated express pipeline:
OTP is generated by your application and passed to the SMS API.
The API immediately classifies the message as OTP priority based on the template category.
The message enters a dedicated OTP queue — separate from all promotional and standard transactional traffic.
The platform connects directly to the carrier's priority API (SMPP or HTTP) — bypassing aggregator queuing layers that add latency.
The carrier delivers the OTP to the subscriber's device.
A delivery receipt (DLR) is returned to your application within seconds.
If delivery fails on the primary route, an automatic failover triggers delivery via a secondary carrier route — without any retry logic on your side.
The total wall-clock time from API call to handset delivery on a well-engineered OTP platform: 1.5 to 3 seconds under normal network conditions.
OTP Security Best Practices
Rate limiting: Implement rate limiting on OTP generation at the application layer. Allow a maximum of 3-5 OTP requests per phone number per hour. Without rate limiting, your OTP system becomes a vector for OTP bombing attacks — where attackers flood a victim's phone with OTP messages to harass them or to probe authentication systems.
Validity window: Never set OTP validity beyond 10 minutes. The shorter the window, the smaller the attack surface for replay attacks. For payment authentication, 3-5 minutes is standard.
One-time use enforcement: Invalidate the OTP immediately after a single successful verification. Do not allow the same OTP to be used more than once.
No forwarding encouragement: Always include "Do not share this code with anyone" in the OTP message body. TRAI guidelines require this warning for financial OTPs.
Masked phone display: When showing users where their OTP was sent, display a masked number (e.g., ******3210) — not the full number — to prevent social engineering.
Voice OTP fallback: For users who cannot receive SMS (network issues, international roaming, feature phones without SMS reception), implement a Voice OTP fallback that calls the number and reads the code aloud via text-to-speech.
OTP SMS Use Cases by Industry
Banking and Finance:
Login OTP for net banking and mobile banking apps
Transaction authorization for IMPS, NEFT, UPI transfers
Card-not-present (CNP) transactions — OTP on registered mobile for online card payments
Password reset and account recovery
New device registration on banking apps
E-Commerce and Payments:
Account creation verification
Guest checkout phone verification
Payment gateway 3D Secure authentication
High-value cart verification before processing
Telecom:
SIM swap verification
Number portability request authentication
Account access on self-care portals
Government / Digital India:
Aadhaar-linked OTP for DigiLocker and UMANG
Income tax filing and GST portal login
EPFO and ESIC portal access
Healthcare:
Patient portal login authentication
Teleconsultation access verification
Insurance claim portal authentication
EdTech:
Student account creation and login
Online exam authentication
Parent portal access verification
Real OTP SMS Examples
Banking Login:
"HDBANK: OTP for login is 847203. Valid for 5 minutes. Do not share. If not you, call 1800-XXX-XXXX immediately. -HDBANK"
Payment Authentication:
"RZRPAY: OTP 391847 for payment of Rs.4,999 to MK Electronics. Valid 3 min. Do not share. -RZRPAY"
App Registration:
"Your TechTo verification code is 726419. Valid for 10 minutes. Do not share this code. -TECHTO"
Account Recovery:
"Your password reset OTP is 584930. Valid for 15 minutes. If you didn't request this, ignore this message. -MYAPP"
4. Flash SMS (Class 0 SMS)
What Is Flash SMS?
Flash SMS — also called Class 0 SMS or Alert SMS — is a special type of text message that appears directly on the recipient's phone screen as a popup alert, without being stored in the SMS inbox unless the recipient manually saves it.
Unlike standard SMS messages that land silently in the inbox, Flash SMS demands immediate attention. The message appears over whatever the user is doing — over their running app, their camera, their phone screen. It cannot be ignored without an action.
This is achieved through the SMS class parameter — a value set in the SMS data coding scheme header. Standard SMS is Class 1 (stored in the inbox). Class 0 means "display immediately and do not store unless instructed."
Technical Mechanism of Flash SMS
When your SMS API sends a Flash SMS, the message payload includes a data coding scheme (DCS) byte that instructs the receiving handset's SMS application to:
Display the message immediately on the active screen as a full-screen or overlay popup.
Do not store the message in the SMS inbox by default.
Wait for user interaction (acknowledge/dismiss) before closing.
The actual delivery path is identical to standard SMS — the Flash classification is entirely a handset-level instruction, not a carrier routing difference.
Compatibility note: Flash SMS behavior varies by handset and operating system. It works reliably on feature phones and most Android devices. Behavior on iOS is more variable — Apple's SMS handling framework does not always respect the Class 0 instruction in the same way as Android. Test Flash SMS behavior across your target device mix before deploying at scale.
Flash SMS Use Cases
Emergency Alerts: Government agencies, disaster management authorities, and public safety organizations use Flash SMS to broadcast urgent public safety messages. A flood warning, earthquake alert, or civil emergency notice that needs immediate visibility on every phone is a prime use case.
Critical Security Alerts: Banks sending fraud alerts for unauthorized card usage. "YOUR CARD WAS USED FOR AN UNUSUAL TRANSACTION. Did you approve Rs.45,000 at 2:41 AM? Call immediately if not." — this needs to be seen before the phone goes into the pocket.
Health and Medical Alerts: Hospitals alerting next-of-kin about a patient's critical condition change. Pharmacies alerting patients about critical drug interaction warnings.
Infrastructure Outage Notifications: Utility companies alerting customers about emergency power cuts, water supply interruptions, or gas leaks in the neighborhood.
Time-Critical OTP Variants: For extremely high-value transactions, some financial institutions use Flash OTP delivery to ensure the OTP is not missed in an inbox full of other messages.
Flash SMS vs. Standard SMS — Key Differences
Dimension | Flash SMS (Class 0) | Standard SMS (Class 1) |
Display behavior | Full-screen popup immediately | Lands in inbox silently |
Stored in inbox | No (unless user saves) | Yes |
User action required | Yes (must dismiss) | No |
Notification sound | Yes (immediate) | Depends on settings |
Best for | Emergency alerts, fraud warnings | All regular messaging |
iOS compatibility | Limited | Full |
DLT classification | Service Implicit (transactional) | Category depends on content |
DLT and Regulatory Considerations for Flash SMS
Flash SMS content is subject to the same DLT registration rules as standard SMS. The DLT category depends on the content — a Flash fraud alert is Service Implicit, while a Flash promotional message (if such a thing were used) would need to follow promotional category rules. In practice, Flash SMS is almost exclusively used for service or emergency messages and is registered under Service Implicit.
5. Service Implicit SMS (Detailed Breakdown)
What Is Service Implicit SMS?
Service Implicit is one of the four DLT regulatory categories and is the primary classification for most transactional messages, OTPs, and all transactional-category alerts. "Implicit" means the customer's consent to receive this type of message is implied by their relationship with the business — no separate explicit opt-in for SMS is required.
When a customer:
Creates an account on your platform,
Makes a purchase,
Initiates a login,
Books an appointment,
Performs a financial transaction,
...they implicitly consent to receive SMS messages about those specific service activities. You do not need a separate "Yes, send me SMS" checkbox because the communication is an inherent, expected part of the service.
What Can Be Sent Under Service Implicit?
OTPs for account authentication and transaction verification
Order confirmation, shipping updates, delivery confirmation
Payment confirmation, receipt, and failure alerts
Bank account debits, credits, and balance alerts
Appointment confirmations and reminders (existing bookings)
Ticket and booking confirmations (flights, trains, hotels)
Account activity notifications (new login from device, password changed)
Subscription activation and renewal confirmation
Utility bill payment confirmations
Government service alerts and notifications linked to citizen services the user is enrolled in
What Cannot Be Sent Under Service Implicit?
Any promotional or marketing content
Cross-sell or upsell offers embedded in otherwise transactional messages
Messages to people who are not existing customers or registered users
General awareness messages not tied to a specific user action or account
The golden rule: If removing the customer's name and account information from the message would make it look like a mass marketing message — it does not belong in the Service Implicit category.
6. Service Explicit SMS
What Is Service Explicit SMS?
Service Explicit is the DLT category for messages sent to customers who have given specific, documented consent to receive a particular type of communication beyond what is implied by their basic service relationship.
Unlike Service Implicit (where consent is implied), Service Explicit requires the customer to have actively opted in to receive these messages. This opt-in must be recorded with a timestamp and source.
Common Service Explicit Use Cases
Health reminders to registered patients: "Your annual health check-up is due. Book at Apollo: bit.ly/hchk" — the patient is registered at the clinic but hasn't recently had an appointment. They opted in to receive health reminders.
Financial advisory updates: "Your mutual fund portfolio report for June is ready. View: mf.link/june" — sent to investors who opted in for monthly portfolio updates.
Subscription content notifications: "New episode of your subscribed podcast is live. Listen: pod.link/ep42" — the user explicitly subscribed to SMS notifications for this podcast.
Alumni and membership communications: University sending placement updates to alumni who opted in for career communications.
Insurance renewal reminders: Policy renewal reminders sent before the expiry date to customers who agreed to receive SMS reminders at the time of enrollment.
Service Explicit DLT Requirements
The message template must be registered under the Service Explicit category.
You must maintain records of when and how each recipient provided their opt-in consent.
Recipients must have a clear, accessible opt-out mechanism.
Message content must stay within the scope of what the user consented to receive.
7. Bulk SMS
What Is Bulk SMS?
Bulk SMS is not a separate DLT category — it is a delivery mode that describes sending large volumes of SMS messages simultaneously. Bulk SMS can be promotional, transactional, OTP, or service messages delivered at high throughput using a batch-capable API or platform.
The defining characteristic of Bulk SMS is scale: instead of sending one message at a time, bulk SMS sends thousands or millions of messages in a single operation, using parallel processing and carrier connections to achieve the required throughput.
Bulk SMS Architecture
A robust bulk SMS platform (like TechTo Networks) handles the following to make bulk delivery reliable:
Queue management: Inbound batch requests are parsed and distributed across multiple carrier connections simultaneously, preventing any single connection from becoming a bottleneck.
Carrier load balancing: Traffic is split across multiple carrier routes in real time, with each route handling a share of the volume. If one carrier slows down, traffic is redistributed.
DND filtering at scale: Before dispatching a promotional bulk batch, the platform automatically filters recipient numbers against the TRAI DND registry, removing non-compliant destinations without any manual effort.
Throttle controls: Some use cases require controlled delivery speed — for example, a contact centre that can only handle 200 inbound calls per hour should not send 100,000 "Call us back" messages in 5 minutes. Throttle controls let you cap delivery speed at the platform level.
Delivery receipt aggregation: Bulk batches generate thousands of individual delivery events. A quality platform aggregates these into batch-level reports (total queued, delivered, failed, DND-filtered) as well as individual message-level DLRs.
Bulk SMS Throughput by Use Case
Use Case | Recommended Throughput | Timing |
Festive promotional campaign | 500-1,000 SMS/sec | 10 AM - 2 PM for peak engagement |
Flash sale announcement | Maximum available | Immediate dispatch at sale start |
OTP batch (multiple users) | 200-500 SMS/sec | Anytime (transactional) |
Appointment reminders | 100-200 SMS/sec | 24-48 hours before appointments |
Payment alerts | Maximum available | Real-time, triggered by events |
Internal staff broadcasts | 50-100 SMS/sec | As needed |
8. API SMS
What Is API SMS?
API SMS refers to SMS messages sent programmatically through an HTTP REST API (or SMPP protocol for high-volume enterprise use) rather than through a web dashboard or CSV upload. API SMS is not itself a DLT category — it is the technical integration method that powers automated, event-driven messaging.
API SMS is the backbone of every automated messaging use case:
OTP triggered when a user clicks "Send OTP" in your app
Order confirmation triggered when a payment gateway confirms payment
Appointment reminder triggered by a scheduler job 24 hours before the event time
Fraud alert triggered when an anomaly detection system flags unusual account activity
REST API SMS vs. SMPP SMS
Most businesses use REST API SMS — simple HTTPS POST requests that any programming language can make. For very high-volume enterprise deployments (1,000+ messages per second sustained throughput), SMPP (Short Message Peer-to-Peer) protocol offers lower latency and higher throughput because it uses a persistent binary TCP connection directly to the carrier — eliminating the overhead of HTTP request setup for each message.
Dimension | REST API (HTTP) | SMPP |
Integration complexity | Low (standard HTTP) | High (binary protocol, persistent connection) |
Latency per message | 80-200ms overhead | 10-30ms overhead |
Max throughput | 100-500 SMS/sec | 1,000-10,000 SMS/sec |
Best for | Most applications | Carrier-grade, ultra-high-volume |
Language support | Any (HTTP) | Requires SMPP library |
Cost | Same per SMS | Usually same + setup |
API SMS Integration Example (Python)
import requests, os
response = requests.post(
"https://api.techtonetworks.com/v1/messages",
json={
"to": "+919876543210",
"from": "TECHTO",
"template_id": "tmpl_order_confirm_001",
"variables": {
"name": "Rahul",
"order_id": "ORD98721",
"amount": "4,999",
"delivery_date": "05-Jul"
}
},
headers={"Authorization": f"Bearer {os.environ['TECHTO_API_KEY']}"}
)
print(response.json())
9. Unicode SMS (Regional Language SMS)
What Is Unicode SMS?
Unicode SMS is any text message that uses characters outside the GSM-7 standard character set. The standard SMS encoding (GSM-7) supports 128 characters — the English alphabet, numbers, and basic punctuation. Unicode (UCS-2) encoding supports the full range of international scripts, enabling SMS in:
Hindi (Devanagari script)
Tamil
Telugu
Kannada
Malayalam
Bengali
Gujarati
Punjabi (Gurmukhi)
Marathi
Odia
Arabic
Chinese, Japanese, Korean
Emoji (any emoji forces Unicode encoding)
Unicode Character Limits
Encoding | Characters | Single SMS | Multi-part SMS segment |
GSM-7 (English) | 128 basic chars | 160 characters | 153 characters |
Unicode (UCS-2) | All scripts + emoji | 70 characters | 67 characters |
A Unicode SMS segment carries less than half the characters of a GSM-7 segment. A 140-character Hindi message would be sent as 3 Unicode segments — each billed individually.
Why Unicode SMS Matters for India
India has 22 official languages and hundreds of regional dialects. For businesses serving Tier 2 and Tier 3 markets, sending SMS in the local language dramatically increases readability, trust, and response rates. A farmer in rural Tamil Nadu who receives a government scheme notification in Tamil will understand and act on it far better than the same message in English.
Under TRAI's Digital India guidelines, government bodies and public sector entities are encouraged to send SMS in regional languages. Private businesses serving regional markets are following suit — and seeing measurable improvements in engagement.
Unicode SMS DLT Registration
Unicode message templates must be registered on the DLT portal in the regional language script. When registering templates, submit the exact Devanagari, Tamil, Telugu, or other script text — not a transliteration in English characters. Most DLT portals now support Unicode input for template registration.
10. Missed Call SMS and Missed Call Alert Services
What Is Missed Call SMS?
Missed Call SMS is a hybrid service where a user gives a "missed call" — calls a designated number and disconnects before the call connects — and receives an automatic SMS response. The missed call itself is the trigger; the SMS is the response.
This service became popular in India because:
Giving a missed call costs nothing for the caller.
It requires no internet, no app, no literacy — just the ability to dial a number.
It works on every mobile phone, including basic feature phones.
It provides a zero-cost way for potential leads to express interest.
How Missed Call SMS Works
A business advertises a 10-digit number in print ads, TV spots, radio, or outdoor advertising. E.g., "Give a missed call to 9876543210 to get our brochure."
A potential customer sees the ad and calls the number.
The system detects the incoming call, rejects it after 1-2 rings (before the caller spends any money).
The system automatically triggers an SMS to the caller's number with the pre-configured response message.
The caller number is logged as a lead in the CRM.
Missed Call SMS Use Cases
Lead generation: "Give a missed call to get our product catalog sent to you." Voter registration and surveys: Government campaigns using missed call as a zero-cost consent signal. Subscription opt-in: "Give a missed call to 9876XXXXXX to subscribe to our daily news digest." Contest entry: "Give a missed call to enter our lucky draw." Service activation: Prepaid telecom services activated by missed call. Feedback collection: "Give a missed call on 9876543210 to rate our service 'Satisfied' or 9876543211 for 'Not satisfied.'"
Missed Call SMS and DLT
The outbound SMS triggered by a missed call is classified as a transactional (Service Implicit) message because it is a direct response to an action initiated by the user. The message template must be registered on the DLT portal under Service Implicit. The sender ID must be a registered 6-character alphanumeric header.
Master Comparison Table — All SMS Service Types
Feature | Promotional | Transactional (Service Implicit) | OTP | Service Explicit | Flash SMS | Bulk SMS | Missed Call SMS |
DLT Category | Promotional | Service Implicit | Service Implicit | Service Explicit | Service Implicit | Depends on content | Service Implicit |
DND Bypass | ❌ No | ✅ Yes | ✅ Yes | ✅ Yes | ✅ Yes | Depends on type | ✅ Yes |
Time Restrictions | 9 AM – 9 PM | None | None | None | None | Depends on type | None |
Sender ID Format | 6-digit numeric | 6-char alphanumeric | 6-char alphanumeric | 6-char alphanumeric | 6-char alphanumeric | Depends on type | 6-char alphanumeric |
Delivery Speed | Standard | Priority | Express | Priority | Priority | Bulk queue | Standard |
Opt-in Required | No (but opt-out) | No | No | Yes | No | Depends | No |
Cost (Approx.) | ₹0.10–0.18 | ₹0.15–0.22 | ₹0.18–0.28 | ₹0.15–0.22 | ₹0.18–0.25 | Volume-based | Custom |
Content Type | Marketing/offers | Service alerts | Auth codes | Service + consented | Emergency | Any | Triggered response |
Reply Supported | No (Promo) | If 2-way enabled | Rarely | If 2-way enabled | No | If 2-way | N/A (missed call) |
Which Type of SMS Service Should You Use? A Decision Framework
Use this decision tree to identify the correct SMS service type for any message you want to send:
Step 1 — Is this message promotional or informational?
Promotes a product, service, offer, or event → Promotional SMS
Delivers information about an existing transaction or service → Go to Step 2
Step 2 — Is the recipient an existing customer who initiated or is part of this transaction?
Yes → Service Implicit (Transactional)
No, but they explicitly opted in to receive this type of communication → Service Explicit
Step 3 — Is the message a one-time code for authentication?
Yes → OTP SMS (use express route, not standard transactional)
No → Standard Service Implicit / Transactional
Step 4 — Does the message need to be seen immediately regardless of what the user is doing?
Yes (emergency, fraud alert, critical security) → Flash SMS (Class 0)
No → Standard delivery
Step 5 — Are you sending to thousands or millions of recipients simultaneously?
Yes → Bulk SMS delivery mode (select the correct content category above and use batch API)
Step 6 — Do you need to reach recipients in regional languages?
Yes → Unicode SMS (applies on top of whichever type you've selected above)
DLT Registration — Step-by-Step for Each SMS Type
All SMS types require DLT registration before commercial use in India. Here is the process:
Step 1 — Register Your Business Entity (PE Registration)
Create an account on your chosen telco's DLT portal (Jio, Airtel, Vodafone Idea, BSNL, TATA). Submit:
Business PAN card
GST registration certificate
Certificate of Incorporation (for companies) or business registration document
Authorized signatory's Aadhaar-linked mobile number
Approval typically takes 2-5 business days. You receive a Principal Entity (PE) ID upon approval.
Step 2 — Register Your Sender ID (Header)
For each SMS type, register the sender ID:
Promotional: 6-digit numeric header (assigned by the portal)
Transactional / OTP / Service Explicit: 6-character alphanumeric (e.g., TECHTO, HDBANK)
One Sender ID can be used for multiple templates within the same category.
Step 3 — Register Your Message Templates
Create a template for each unique message structure. Rules:
Static text is written exactly as it will appear.
Dynamic fields (name, OTP, order ID, amount) are marked as {#var#}.
Select the correct category: Promotional, Service Implicit, Service Explicit, or Transactional (legacy).
Template content must match the category — no promotional language in transactional templates.
Example DLT Template (Transactional — OTP):
"Your OTP for {#var#} is {#var#}. Valid for {#var#} minutes. Do not share. -{#var#}"
Template approval takes 24 to 72 hours on most DLT portals.
Step 4 — Link Templates to Your SMS Provider
Once approved, share your PE ID, Sender ID, and Template IDs with your SMS provider (TechTo Networks). The provider links these registrations to your account so that all messages sent through the API automatically include the correct DLT-registered template ID in the carrier payload.
Step 5 — Maintain and Audit Templates
Review registered templates quarterly. Archive unused templates. Update templates via the DLT portal when your message structure changes — never send a message that deviates from the registered template format, even slightly.
Industry-Wise Recommended SMS Service Types
E-Commerce and D2C Brands
Use Case | SMS Type |
Sale and offer announcements | Promotional |
Order confirmation | Service Implicit (Transactional) |
Shipping dispatch | Service Implicit |
Out for delivery | Service Implicit |
Delivery confirmation | Service Implicit |
Return pickup scheduled | Service Implicit |
Login OTP | OTP |
Payment authentication | OTP |
Loyalty offer to opt-in subscribers | Service Explicit |
Banking, Finance, and Insurance (BFSI)
Use Case | SMS Type |
Transaction debit/credit alert | Service Implicit |
Low balance notification | Service Implicit |
Login OTP | OTP |
Card transaction OTP | OTP |
New device OTP | OTP |
Loan EMI reminder | Service Explicit |
Policy renewal reminder | Service Explicit |
Credit card offer | Promotional |
Fraud alert | Flash + Service Implicit |
Healthcare
Use Case | SMS Type |
Appointment confirmation | Service Implicit |
Appointment reminder (24hr) | Service Implicit |
Lab report ready | Service Implicit |
Patient portal login OTP | OTP |
Health check-up reminder | Service Explicit |
Wellness tips (opted-in) | Service Explicit |
Awareness campaigns | Promotional |
Emergency alerts | Flash SMS |
EdTech and Education
Use Case | SMS Type |
Fee payment confirmation | Service Implicit |
Exam schedule | Service Implicit |
Result notification | Service Implicit |
Login OTP for exam portal | OTP |
Admission campaign | Promotional |
Scholarship deadline reminder | Service Explicit |
New batch announcement | Promotional |
Logistics and Supply Chain
Use Case | SMS Type |
Pickup confirmation | Service Implicit |
Shipment dispatched | Service Implicit |
Out for delivery alert | Service Implicit |
Delivery confirmation + OTP | OTP |
Delivery failed (rescheduling) | Service Implicit |
Driver arrival alert | Service Implicit |
Post-delivery feedback request | Service Explicit |
Common Mistakes Businesses Make with SMS Types (and How to Avoid Them)
Mistake 1: Mixing promotional and transactional content Adding "Use code WELCOME10 for 10% off your next order" to an order confirmation message. This single sentence reclassifies the entire message as promotional — making it a DLT violation and blocking it from reaching DND-registered customers. Fix: Keep transactional and promotional messages strictly separate. If you want to upsell, send a separate promotional message.
Mistake 2: Using promotional routes for OTP OTP messages sent via promotional routes are queued behind thousands of marketing messages and may take 30 seconds to several minutes to deliver. For time-sensitive authentication, this is catastrophic. Fix: Always use dedicated OTP or transactional routes for authentication messages.
Mistake 3: Sending promotional messages outside 9 AM–9 PM Many businesses schedule batch sends without checking dispatch time carefully. A batch queued at 8:30 PM may still be dispatching at 9:05 PM — resulting in carrier-blocked messages and wasted budget. Fix: Always schedule promotional batches to complete dispatch by 8:45 PM to account for queue processing time.
Mistake 4: Not filtering DND numbers from promotional lists Sending promotional messages to DND-registered numbers triggers TRAI complaints and can result in sender ID blacklisting. In bulk operations, even a small percentage of DND violations can cause platform-level penalties. Fix: Use a provider (like TechTo Networks) that automatically filters DND numbers from all promotional dispatches.
Mistake 5: Unregistered or mismatched templates Sending a message whose content does not match the registered DLT template — even a minor word change — causes carrier-level rejection. The message is blocked silently, and you have no delivery report to debug against. Fix: Always pass the template_id in your API call. TechTo Networks validates template match before dispatch, returning a clear API error if there is a mismatch — before the message costs you anything.
Mistake 6: Not having a voice OTP fallback Pure SMS-only OTP delivery fails when users have no signal, are roaming internationally, or have full SMS inboxes. Without a fallback, they cannot authenticate — and they churn. Fix: Implement Voice OTP as a fallback channel. After 15-30 seconds without a delivered OTP, automatically trigger a voice call that reads the code aloud.
Frequently Asked Questions
What is the difference between Promotional SMS and Transactional SMS? Promotional SMS is used for marketing, offers, and advertising — sent only during 9 AM to 9 PM, cannot reach DND-registered numbers, and uses a 6-digit numeric sender ID. Transactional SMS (Service Implicit) delivers essential service information tied to a customer's account or transaction — can be sent 24/7, reaches DND numbers, and uses a 6-character alphanumeric sender ID. The content determines which category applies.
Can I send promotional SMS to DND-registered numbers? No. Promotional SMS cannot reach DND-registered numbers under TRAI regulations. Attempting to send will result in messages being blocked at the carrier level. Your SMS provider should automatically filter DND numbers from your promotional dispatch lists. Transactional and OTP messages can reach DND numbers.
What is the DLT category for OTP messages? OTP messages are classified under Service Implicit — the same category as standard transactional messages. However, OTP messages are typically routed through dedicated express carrier pathways separate from the standard transactional queue, ensuring sub-5-second delivery.
What happens if I send a promotional message using a transactional sender ID? This is a TRAI violation. Carriers detect category mismatches between the sender ID category and message content, and can block the message, flag your account, or blacklist your sender ID. The DLT framework is designed specifically to prevent this kind of misuse.
What is a DLT template and why is it mandatory? A DLT template is a pre-registered message structure approved by your telecom operator. All commercial SMS in India must be sent using an approved template. The template defines the fixed content and marks dynamic parts (like names and OTPs) with {#var#} placeholders. Messages sent without a matching registered template are blocked by carriers.
How long does DLT registration take? Business entity (PE) registration typically takes 2-5 business days. Sender ID and template approval takes 24-72 hours per submission on most DLT portals. TechTo Networks provides guided DLT registration support to help businesses complete the full process efficiently.
Can I use an alphanumeric sender ID for promotional SMS? No. Under TRAI rules, promotional SMS must use a 6-digit numeric sender ID. Only transactional, OTP, and service-category messages can use 6-character alphanumeric sender IDs (brand names). This distinction is enforced at the DLT registration level.
What is Flash SMS and when should I use it? Flash SMS (Class 0 SMS) is a message that appears as an immediate popup on the recipient's screen — bypassing the inbox entirely. It is used for emergency alerts, critical fraud warnings, and time-sensitive security notifications where the message must be seen immediately. It requires user acknowledgment before it closes.
What is the character limit for a regional language SMS in Hindi? Hindi uses Devanagari script, which requires Unicode (UCS-2) encoding. A single Unicode SMS segment carries 70 characters. Messages longer than 70 characters are split into multi-part segments of 67 characters each. A 200-character Hindi message would be sent as 3 Unicode segments, with each billed separately.
What is Service Explicit SMS and when is it required? Service Explicit is used when you want to send messages to customers beyond what their basic service relationship implies — and they have explicitly consented to receive those specific communications. Examples: monthly portfolio statements to investment customers who opted in for SMS updates, health reminders to clinic patients who agreed to receive periodic wellness SMS. You must maintain documented consent records.
How is OTP SMS different from regular Transactional SMS technically? Both are classified as Service Implicit under DLT. The technical difference is in routing: OTP messages are sent through dedicated express carrier pathways with sub-5-second delivery guarantees, separate from the standard transactional queue. Standard transactional messages may take 5-15 seconds. For authentication use cases, the speed difference is critical.
Can I send Unicode (regional language) templates through DLT? Yes. DLT portals support Unicode template registration in regional scripts. Submit your template in the actual script (e.g., Devanagari for Hindi, Tamil script for Tamil) — not in English transliteration. The same DLT rules apply: static content is fixed, dynamic fields use {#var#} placeholders.
What is missed call SMS and how does it work? A missed call SMS service assigns a dedicated 10-digit phone number to a campaign. When a user dials that number and disconnects (before it connects, so no cost to the caller), the system automatically sends a pre-configured SMS to the caller's number. It is widely used for lead generation, opt-in collection, and zero-cost engagement. The outbound SMS is registered as a Service Implicit transactional message.
How do I choose between Promotional and Service Explicit for a subscription communication? If you send content communications to subscribers (news digests, fitness tips, content updates), classify them as Service Explicit — the subscriber explicitly opted in for this specific type of communication. If your message is a discount, sale, or marketing offer, it is Promotional regardless of the user's subscription status for content.
What is OTP bombing and how do I prevent it? OTP bombing is an attack where an automated script sends repeated OTP requests to a target phone number, flooding the victim with messages. Prevent it by: rate limiting OTP generation to 3-5 requests per number per hour, implementing CAPTCHA before OTP triggers on web forms, alerting users when unusual OTP request frequency is detected, and blocking automated OTP triggers from suspected bot IPs.
Summary — Types of SMS Services at a Glance
India's SMS ecosystem has eight distinct service types, each governed by specific TRAI rules, DLT categories, and technical routing requirements. Getting the right type for the right use case is not just a compliance obligation — it directly affects your delivery rates, costs, and the customer experience.
Promotional SMS is your marketing channel — affordable, reach-limited by DND and time windows, designed for mass outreach and offers.
Transactional SMS (Service Implicit) is your service communication backbone — highest trust level, 24/7, DND bypass, alphanumeric sender ID, for all account and service-related messages.
OTP SMS is your authentication fast lane — express routing, sub-5-second delivery, the mandatory choice for any verification code.
Service Explicit SMS is your consent-based service communication channel — for periodic updates, reminders, and content to customers who opted in specifically.
Flash SMS is your emergency interrupter — for messages that must be seen immediately, above everything else on the screen.
Bulk SMS is your delivery scale engine — the mode for sending any of the above types to thousands or millions of recipients simultaneously.
Unicode SMS is your regional reach multiplier — enabling all the above types in any Indian script or international language.
Missed Call SMS is your zero-cost engagement trigger — turning a free caller action into an automated, compliant outbound SMS response.
TechTo Networks supports all eight SMS service types from a single platform — with automated DLT compliance, dedicated OTP routing, DND filtering, multi-carrier failover, and real-time delivery reporting. Every type, every compliance requirement, one dashboard.
Great breakdown of how SMS services can improve customer communication for businesses. The explanation of transactional and promotional SMS was especially useful for growing brands.