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Ti+  for  TS 24.228§7.2.2§7.3.2§7.4.2
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IMS S1 signalling flow - Session Initiation: Mobile origination and termination roaming, with different network operators - IMS S1-7a. SIP 180 Ringing response (INVITE): from UE2 to P-CSCF2
  • 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.