SMS not being received by user even after a successful 'Message has been accepted by phone'

Question

Using Amazon SNS for transactional messages. One of our users is not receiving the SMS always - only receives it intermittently.

Our CloudWatch logs say "Message has been accepted by phone" for the phone number and the phone number is being sent with the correct area code.

What could be going wrong? How do we diagnose further?

Thanks.

Answer

Phone carrier issues - Even though SNS marks the message as delivered, the final delivery depends on the phone carrier. Carriers may temporarily block messages if they detect abnormal traffic patterns.

Sender ID not registered - For some countries like India, the sender ID (registered with the carrier) needs to match the phone number used to send SMS. Without proper registration, carriers may intermittently accept messages.

Downstream provider/aggregator issues - Issues at providers that route traffic between SNS and carriers could cause intermittent failures.

To diagnose further:

Check CloudWatch logs for failure reasons like

```
PhoneNumberOptedOut
```

or

```
ThrottlingLimitExceeded
```

.

Contact AWS Support with recent message logs. They can work with providers to identify issues.

Ensure sender ID setup and templates are correctly configured if required for the destination country.

Monitor SMS usage to avoid hitting service quotas which force low priority.

SMS not being received by user even after a successful 'Message has been accepted by phone'

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