Short answer
Users usually miss OTP SMS for one of a few reasons: the number is wrong or badly formatted, the phone is off or out of coverage, the sender ID is not registered for that country, an operator filter blocks the message, or the user blocked promotional senders. Delivery reports show which cause applies, so check them first.
Common causes and how to spot them
| Cause | Typical signal |
|---|---|
| Wrong or non-E.164 number | Rejected or failed status straight away |
| Phone off, roaming or out of coverage | Expired status after the validity period |
| Unregistered sender ID | Failures concentrated in one country or operator |
| Operator filtering | Rejected or undelivered on one network, often after content changes |
| Handset blocking or spam folder | Delivered status, but the user sees nothing |
How to troubleshoot OTP delivery
- Look up the delivery report for the affected numbers and note the status and error code.
- Check the number is in full international (E.164) format.
- Confirm the sender ID is registered for the destination; in GCC markets sender IDs are typically registered before use.
- Keep OTP text short and free of marketing content or unknown short links.
- Test on each major operator in the market.
- Add a fallback: WhatsApp authentication template or a voice call after a short timeout.
Limits of delivery reports
Some operators return receipts late or not at all, and some report delivered when the message reached the network rather than the phone. Treat a delivery report as strong evidence, not proof. A sudden fall in codes entered while send volume rises can also mean SMS pumping, not a delivery problem.
OmniChannels.ai provides delivery-status webhooks, HLR number lookup and OTP fallback across SMS, WhatsApp and voice.