Every time you log in to a banking app, place an order or reset a password, you receive a short code on your phone – a one-time password. For businesses, sending these codes reliably is critical: if the code is late, the customer cannot sign up, log in or pay. This guide explains what OTP SMS is, how it works in India, the DLT rules you must follow and the best practices for fast and secure OTPs.
What is OTP SMS?
An OTP (one-time password) is a short numeric code, usually 4 to 6 digits, that is valid for a single use and for a short time. An OTP SMS is the text message that delivers this code to the customer's mobile number. Because only the person holding the phone can see the code, it is a simple way to confirm that a mobile number is real and belongs to the user.
Where are OTP messages used?
- Sign-up and mobile number verification
- Login and two-factor authentication (2FA)
- Password reset
- Payment and transaction confirmation
- Cash-on-delivery order confirmation
- Account changes such as updating bank details or email
How does OTP SMS work?
- The customer enters a mobile number in your app or website.
- Your server generates a random code and stores it with an expiry time.
- Your server calls the SMS API with the number and the message.
- The SMS provider routes the message to the mobile operator, which checks it against your DLT template and delivers it.
- The customer types the code, and your server checks that it matches and has not expired.
All of this should take only a few seconds. Slow or failed OTP delivery directly increases drop-offs at sign-up and checkout.
DLT rules for OTP SMS in IndiaAn OTP is a transactional messagee, so it is delivered 24×7 and even to numbers on the DND list. But it must follow DLT registration rules like every other commercial SMS:
- Register your business (PE ID) and a sender ID (header) on a DLT portal.
- Register an OTP template in the transactional category with your brand name in it.
- Send exactly the approved text – only the variable parts (like the code) can change.
Example OTP template:
{#var#} is your OTP to log in to Digital SMS 4 U. It is valid for 10 minutes. Do not share it with anyone.Best practices for sending OTPsS
1. Use a dedicated transactional route
Never send OTPs on a promotional route. Promotional messages are blocked for DND numbers and outside allowed hours. A good OTP SMS service uses priority transactional routes for fast delivery.
2. Keep the message short and clear
Put the code at the start, mention your brand and the validity, and add a warning not to share it. Keep it within one SMS (160 characters in English) to save cost and avoid delays.
3. Set a short expiry
Codes should expire in a few minutes (for example 5–10 minutes) and become invalid after one successful use.
4. Limit attempts and resends
Allow only a few wrong attempts and add a waiting time before "Resend OTP". This stops attackers from guessing codes and protects your SMS balance from abuse by bots.
5. Protect your OTP form from bots
Use captcha or rate limits on the "Send OTP" button. Fake requests to random numbers (SMS pumping) can waste a large amount of credits in minutes.
6. Never show or log the OTP
Do not return the code in your API response or print it in logs. Store only what you need to verify it, and delete it after use.
7. Support Android auto-read
On Android, a short app hash at the end of the message lets your app read the OTP automatically, so the customer does not have to type it. Register this format in your DLT template.
8. Monitor delivery reports
Track delivery time and failure rate per operator. If delivery slows down, you need to know before your customers complain.
9. Keep a backup channel
For important flows, offer a fallback such as a voice call OTP or WhatsApp authentication message if the SMS does not arrive.How to choose an OTP providerr
Look for fast delivery on all networks, a simple API with clear documentation, real-time delivery reports, DLT support and support that responds quickly. Read our checklist for choosing an SMS provider for details.Send OTPs with Digital SMS 4 UU
Digital SMS 4 U offers a fast OTP SMS service with transactional routes, a developer-friendly API, DLT help and live delivery reports. Contact us for a free demo, or see our pricing.
Frequently asked questionsAre OTPs delivered to DND numbers??
Yes. OTPs are transactional messages, so they are delivered to DND numbers, 24 hours a day, as long as they use an approved DLT template.How fast should an OTP arrive??
Ideally within a few seconds. If OTPs regularly take longer, check the route and the provider's delivery reports.
How many digits should an OTP have?
4 to 6 digits is common. Six digits are more secure for login and payments.Can I send OTPs without DLT??
No. OTP messages without a registered header and template are blocked by Indian operators.
