Before proceeding with session establishment, UE2 waits for two events. First, the resource
reservation initiated previously must complete successfully. Second, the resource reservation initiated
by the originating endpoint must complete successfully (which is indicated by message UPDATE received by
UE2). UE2 may now immediately accept the session or alert the
destination subscriber of an incoming session attempt; if the latter it indicates this to the calling party by
a 180 Ringing provisional response sent to P-CSCF2.