the value if timerY <IST> After having sent CANCEL the INVITE
transaction also times out -
However, a UAC canceling a request cannot rely on receiving a 487
(Request Terminated) response for the original request, as an
RFC 2543-compliant UAS will not generate such a response.
If there is no final response for the original request in
64*T1 seconds (T1 is defined in Section 17.1.1.1), the client
SHOULD then consider the original transaction cancelled and
SHOULD destroy the client transaction handling the original request.