Callback to Out-of-Session handler returned a non-success status code.
Possible Causes
Your Out-of-Session callback URL is invalid.
Possible Solutions
Verify that your Out-of-Session callback URL is valid and that your server is correctly processing Out-of-Session callbacks from Proxy.
Please read over the [Out-Of-Session Callback guide] (https://www.twilio.com/docs/proxy/out-session-callback-response-guide) for more detailed information.