SIP CANCEL

A SIP method that aborts a pending INVITE transaction before a final response is received, used when the caller hangs up during ringing. CANCEL must match the CSeq and Call-ID of the INVITE it cancels. On receiving CANCEL, the server responds 200 OK to the CANCEL and sends a 487 Request Terminated to the original INVITE. CANCEL cannot be used after a 200 OK has been received — only BYE can terminate an established dialog.