13801: Refer not allowed on non-SIP call legs

The Refer verb is only allowed on SIP call legs, e.g. you can refer an incoming SIP call that originate from SIP infrastructure or an outbound call to a SIP destination. Calls to or from other destinations, such as a phone number or Twilio Client cannot be referred.

• Your application attempted to invoke the Refer verb on a PSTN or Twilio Client call leg.

• For more information on understanding what does and does not constitute a SIP call leg, please see Understanding SIP Call Legs.
• While invoking Refer on a SIP leg is unique in that it removes Twilio from the call path, the effect of transferring a call to another party can be accomplished on all leg types by simply using the Dial verb to call the other party.