When you stop receiving incoming SMS in your salesforce CRM, it might be because of below error from your salesforce end -:

'[{"message":"TotalRequests Limit exceeded.","errorCode":"REQUEST_LIMIT_EXCEEDED"}]'

You can check the API Requests from your salesforce Setup->Administration Setup->Company Profile->Company Information

API Requests, Last 24 Hours , if the number there is exceeding MAX number in parenthesis, you get this error.

You either have to wait till that number goes down or purchase more licenses to raise the MAX limit if you expect the usage to always be higher that what your current MAX limit is.