13750: Twiml verb not supported by this API version.

This error occurs when Twilio starts a new TwiML session with an apiVersion that does not support the TwiML your Voice Application returns. The Application resource uses apiVersion to start the new TwiML session, and this error directs you to use 2010-04-01 or later.

• The TwiML Application is configured with apiVersion 2008-08-01, so Twilio starts the call with an older TwiML session version.
• The TwiML returned by your webhook uses a verb that requires 2010-04-01 or later.

• Update the Application resource apiVersion to 2010-04-01.
• If you must keep the current API version, return only TwiML verbs supported by that version.
• Confirm the call is reaching the intended Voice URL and that the response is valid TwiML for the selected API version.

• REST API: Applications
• TwiML™ for Programmable Voice