Question
What is the proper method to cancel a transfer request that has been initiated with a SIP device?
Product
- Programmable Voice
- Elastic SIP Trunking
Answer
SIP does not have a built-in mechanism to cancel a REFER request once it has been sent; there is no SIP method to retract or undo a REFER after the recipient receives it. While the sender of the REFER can end the related subscription early by unsubscribing, stopping the subscription—either by sending an unsubscribe request or rejecting NOTIFY messages—does not actually cancel or withdraw the REFER request itself.
If the sender (requestor) wants to stop receiving NOTIFY messages, they can do so by sending a SUBSCRIBE request with Expires: 0 to the same dialog.
The only effective way to cancel a SIP REFER request is to terminate (hang up) the call leg that initiated the REFER before the transfer is completed. If the originating party ends the call before the REFER process is finished, the REFER is considered canceled.
Note: Twilio does not support accepting SIP SUBSCRIBE requests for event notifications (such as presence or dialog state). While Twilio does support SIP REFER for call transfers, it does not handle incoming SIP SUBSCRIBE requests but do send NOTIFY requests.
Additional Information