[Sinch developers docs, troubleshooting]: If you have multiple service plans, check the service plan ID in the call matches the credentials used. If testing, the destination number must be verified in the Sinch Build Dashboard. Ensure you are sending from a Sinch number (your test number or a rented one). Confirm the callback URL is correctly configured, required parameters are present, and SDK dependencies are referenced. When in doubt, look up the HTTP error code in the status-codes table before changing config.

Context: Sinch SMS API calls authenticate but test messages never go out. Sinch documents a troubleshooting checklist for this exact case.

## Matched source
Source: Source: https://developers.sinch.com/docs/sms/api-reference/troubleshooting
Original query: "Sinch SMS: test messages fail - verify number and sender requirements"
Key terms: fail, messages, number, requirements, sender, sinch, test, verify