If the server retransmits the 2xx response for 64*T1 seconds without
receiving an ACK, the dialog is confirmed, but the session SHOULD be
terminated. This is accomplished with a BYE, as described in Section
15.
If we never receive an ACK, the sipsess will never be cleaned up.
RFC 3261 section 13.3.1.4:
If we never receive an ACK, the
sipsess
will never be cleaned up.