When users complain “I didn’t get the OTP”, the problem can be anywhere in the chain: your application, the SMS gateway, the route, the operator or the handset. The good news is that most OTP problems fall into a few common categories.
1. Sender ID Not Allowed or Filtered
In many countries, SMS firewalls enforce strict rules on sender IDs. If you use an unapproved sender name or an international sender for domestic traffic, messages may be silently dropped.
Fix:
- Use operator-approved sender IDs for transactional/OTP traffic
- Avoid changing sender IDs frequently
- Check with your gateway about local regulations for each country
2. Low-Quality or Grey Routes
Not all SMS routes are equal. Low-cost “grey routes” may work for some traffic but often fail or delay time-sensitive OTPs.
Fix:
- Use A2P-grade routes specifically for OTP and important alerts
- Monitor DLR stats by route and cut off poor performers
- Maintain at least one backup route per country/operator
3. Subscriber Issues (DND, Out of Coverage, Switched Off)
Sometimes the network and routes are fine but the phone cannot receive messages because it is:
- In DND or blocked for promotional SMS
- Out of network coverage for a long time
- Switched off or in airplane mode
Fix:
- Use transactional routes for OTP that bypass DND where legally allowed
- Implement retries with reasonable timeouts
- Offer alternate channels such as email OTP or voice call
4. Content and Template Problems
Firewalls and spam filters analyze message content. Overly promotional messages, suspicious links or mismatched templates can cause blocks.
Fix:
- Keep OTP templates simple and consistent
- Avoid unnecessary links and promotional lines in OTP messages
- Register templates when required by local regulations
Example safe OTP template:
Your OTP is 453221. Do not share this code with anyone.5. Application and API Issues
Sometimes the problem is not in the route but in the way your system sends messages—for example, rate limits, wrong parameters or missing country codes.
Fix:
- Validate phone numbers and always include country code (+880, +91, etc.)
- Log every API request and gateway response for troubleshooting
- Apply reasonable rate limits and queue management for high load events
6. SMPP Throttling and Queue Overload
If you connect via SMPP, sending too fast without respecting throttle limits can result in rejects and delays. If your gateway queue is overloaded, OTPs may time out.
Fix:
- Align your sending speed with operator/gateway throughput limits
- Prioritize OTP traffic over promotional campaigns in your queue
- Monitor queue size and system load on your SMS platform
7. Handset and Inbox Problems
On the user’s device, a full inbox, misconfigured SMS app, or blocked numbers can also prevent OTP from appearing—even if it was technically delivered.
Fix (User Side):
- Ask users to clear old SMS if inbox is full
- Check if your sender is blocked or in spam list
- Try inserting the SIM into another phone as a test
How to Improve OTP Success Rate
- Use separate, high-quality routes for OTP vs. bulk marketing
- Monitor DLR metrics per country, operator, sender and template
- Implement retry logic and escalation to backup channels (email/voice)
- Work with a provider that understands telecom routing and regulations
OnnetBD IT & OTP Messaging
OnnetBD IT designs OTP delivery systems with:
- Multi-route architecture tuned for delivery speed
- DLR logging and analytics to detect problems early
- REST and SMPP interfaces for your applications
- Integration with your existing login or transaction flows
👉 If OTP delivery is hurting your business, reach out to OnnetBD IT and we’ll help analyze your routes and platform to boost reliability.