During an outbound call using Plivo, if there are any HTTP API error response it means that your request is unable to be processed so as to execute the call. During such cases, calls will not be queued and you will not be charged.
Below are the error responses which you may come across while triggering an outbound call.
400 - A parameter is missing or is invalid
401 - Authentication failed
404 - Resource cannot be found
405 - HTTP method not allowed
- 500 - Server error