Amazon SNS SMS not delivered to China Mobile Hong Kong (CMHK) numbers

The issue you're experiencing with SMS delivery failures to China Mobile Hong Kong (CMHK) numbers while other Hong Kong carriers work fine is a pattern that has been observed with carrier-specific delivery issues in Amazon SNS.

There are several potential reasons for this problem:

1. Carrier-specific restrictions: China Mobile may have implemented specific filtering or blocking mechanisms that affect the delivery of messages from AWS SNS. This could be due to security policies, anti-spam measures, or other carrier-specific regulations.

2. Regional connectivity issues: There might be connectivity problems between AWS SNS in the Singapore region and China Mobile Hong Kong network nodes.

3. Message content or format: There might be additional requirements or restrictions specific to China Mobile Hong Kong that are not being met by your messages.

4. Routing or aggregator issues: The problem could lie with the downstream providers or aggregators that AWS uses to route messages to China Mobile Hong Kong.

To troubleshoot this issue:

1. Double-check your message content to ensure it complies with all regulations and doesn't contain any content that might trigger filtering.

2. Verify that your sender ID and templates are correctly set up for Hong Kong.

3. Enable detailed CloudWatch Logs for your SMS messages to gather more information about the delivery attempts and any specific error codes. Look for log group "sns/region/AccountID/DirectPublishToPhoneNumber/Failure" to see detailed failure information.

4. Monitor your sending rates and ensure you're not exceeding any quotas that might affect message priority.

5. Consider testing with different message types (promotional vs. transactional) to see if that affects delivery.

If the issue persists, you might want to consider opening a support ticket with AWS, as they would have more detailed insights into carrier-specific issues and could potentially work with their providers to resolve any routing or connectivity problems with China Mobile Hong Kong.
Sources
SNS SMS "Unknown error attempting to reach phone" only for China Mobile numbers | AWS re:Post
AWS shows that the SMS was sent successfully, but the user did not receive it | AWS re:Post
AWS SNS can not send SMS to mobile phone | AWS re:Post
Setting SMS messaging preferences in Amazon SNS - Amazon Simple Notification Service