Voice Conversation: Callback event response error.

Request to status callback URL received a response error from the customer-side server.

• This error indicates that Twilio was able to reach the customer-provided URL, however, received an error from the customer-side callback endpoint.
• The URL provided was likely valid, but Twilio might be receiving an error from the customer-side service.

• Make sure you have a web server up and running that serves that URL with the configured method in TwiML, e.g. POST.