Normally our application is sending transactional SMS via us-west-2 and us-east-1. Sending limits are not exceeded. Some days ago sending transactional SMS stopped working. About 12 hours later the AWS CloudWatch logs show the error: "Unknown error attempting to reach phone". The phone numbers received SMS successfully in the past. The receivers are located in Germany. Unfortunately, the API's response code is 200 (success), even though the SMS is not sent successfully.
Very surprisingly, some SMS is sent successfully. But most SMS are resulting in the before mentioned error. Even sending transactional SMS via AWS SNS Console are not working. The green Flash-Message is indicating a success, however, no SMS is sent.
What is causing this error? Are there possibilities to detect failed SMS in our application?