Dial: No referUrl attribute specified

In order to send a SIP REFER message to an established <Dial> call, the referUrl attribute must be specified in the TwiML. In this case, a REFER was received on a <Dial> call without a referUrl attribute present.

• Your <Dial> verb did not include a referUrl attribute or the attribute was misspelled

• Add a valid referUrl attribute to your <Dial> verb
• For more information on sending REFER to Twilio, see Refer to Twilio