While triggering an API request for outbound call/SMS or when the application is executed during an inbound call/SMS if you face any 4XX error or 5XX error, then you can understand that your application could be causing an issue.
Below are the typical error log and their description
400 - A parameter is missing or is invalid
401 - Authentication failed
404 - Resource cannot be found
405 - HTTP method not allowed
- 500 - Server error