- Before proceeding with session establishment, UE2 waits for two events. First, the resource reservation initiated in step #4 must complete successfully. Second, the resource reservation initiated by the originating endpoint must complete successfully (which is indicated by message UPDATE received by UE1). UE2 may now alert the destination subscriber of an incoming session attempt; it indicates this to the calling party by a 180 Ringing provisional response sent to P-CSCF2.