The request method CANCEL is used to cancel pending transactions.
System Specific Information
Pending transactions can be:
The calling UA (User Agent) hangs up after called UA begins ringing, but does not answer.Parent topic: SIP Overview
The calling UA sends a CANCEL request.
The called UA receives a CANCEL request, stops ringing and sends a “200 OK” response and a “487 Transaction Canceled” response.
The calling UA finishes “three-way handshake” by sending an ACK request.