Error message: No origination entities available to send when trying to verify a GB mobile from SNS in the Sandbox

The error messages you're encountering suggest that there's an issue with the origination identity setup for sending SMS messages to UK (GB) numbers in your AWS account. Here are some suggestions to resolve this issue:

1. Sender ID vs. Origination Number: While you've set up a Sender ID, it's important to note that in some regions, including the UK, a Sender ID alone may not be sufficient. You might need an actual origination phone number.

2. Region-specific requirements: Ensure that your Sender ID or origination number is compatible with sending messages to UK numbers. The origination identity you're using must have the capability to send messages to the country you're targeting (in this case, the UK).

3. Verify origination identity: Double-check that your origination identity (whether it's a Sender ID or a phone number) is properly registered and active in your account for the eu-west-2 region.

4. Phone number format: Ensure you're entering the UK phone number in the correct format, including the country code (e.g., +44).

5. Waiting period: If you've recently made changes to your account or origination settings, it may take some time for these to propagate through AWS systems.

6. Use simulator numbers: For testing purposes, you could try using AWS-provided simulator phone numbers. These don't require verification and can help isolate whether the issue is specific to your actual phone number or a more general configuration problem.

7. Check other regions: As a troubleshooting step, you could try setting up in a different region (e.g., us-east-1) to see if the issue persists.

If you've verified all these points and are still encountering the error, it would be advisable to contact AWS Support. They can provide more specific guidance based on your account configuration and use case, especially since you've already gone through the SMS End User Messaging Quick Start SMS Setup Guide.
Sources
Amazon SNS - Unable to verify my phone number | AWS re:Post
SNS sandbox testing using multiple phone numbers | AWS re:Post
Error when adding a Destination Phone Number to SMS simulator "Conflict Occurred" | AWS re:Post