This message means your server or application is unaccessible or failing to load, and Tropo is getting back a standard HTTP status code. These codes are described in numerous locations online, here's one example:
Common ones are:
301 / 307 - Moved / Redirect: This means the URL you defined for your application is redirecting to another URL immediately; per the standards, a POST cannot be redirected and our JSON request is sent with a POST.
400 - Bad Request: This typically means there's a syntax error on the recipient side, so no valid response is returned. Seems to usually show up with PHP applications that have bad syntax somewhere in the code.
404 - Not Found: The URL defined for the application cannot be reached.
405 - Method Not Allowed: We're sending a POST, but the server at the defined URL is set up to only accept a GET (or in rarer cases, only set up to accept a PUT).
500 - Internal Server Error: Also returned when bad syntax is detected in the application code and no valid response is returned to Tropo.
503 - Service Unavailable: This typically means the URL is valid, but there's nothing loading. If using a cloud service, the service might be down or timing out when Tropo sends JSON to it.