AWS shows that the SMS was sent successfully, but the user did not receive it

Based on the information provided, it appears that the issue may not be directly related to AWS or your account setup, but rather could be due to carrier-specific factors or restrictions. Since SMS messages are being successfully delivered to some operators (Giga, M1, Simba) but not to Singtel, this suggests a potential issue with how Singtel is handling or filtering messages from AWS SNS.

Several factors could contribute to this situation:

1. Carrier restrictions: Singtel may have implemented specific filtering mechanisms or security policies that are preventing the delivery of SMS messages from AWS SNS.

2. Sender ID registration: Some countries require sender IDs to be registered with carriers. It's possible that your sender ID is not properly registered or recognized by Singtel.

3. Message content: The content of your messages might be triggering spam filters specifically on Singtel's network.

4. Network issues: There could be temporary network issues or routing problems between AWS and Singtel.

To troubleshoot and resolve this issue:

1. Contact Singtel directly to inquire about any restrictions or requirements for receiving SMS messages from cloud services like AWS SNS.

2. Verify if there are any specific sender ID registration requirements for Singtel in Singapore.

3. Review your message content to ensure it complies with all carrier guidelines.

4. Enable and analyze Amazon CloudWatch Logs for your SMS messages to get more detailed information about the delivery attempts and any potential issues specific to Singtel numbers.

5. If the problem persists, consider opening a support ticket with AWS. They may be able to provide more specific insights or work with their SMS providers to identify any issues in message routing to Singtel.

Remember, even though AWS shows the SMS as sent successfully, the final delivery depends on the receiving carrier's systems and policies. The discrepancy between AWS logs and actual receipt could be due to how different carriers handle and report message statuses.

Sources
AWS SNS can not send SMS to mobile phone | AWS re:Post
SMS not being received by user even after a successful 'Message has been accepted by phone' | AWS re:Post
AWS SNS - SMS not receive to mobile at some times | AWS re:Post