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TS 24.228 – §7.2.3 – §7.3.6 – §7.4.4   List   Prev   Next
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IMS S4 signalling flow - Session Initiation: Mobile origination in home network, Termination in CS network - IMS S4-7a. SIP 180 Ringing response (INVITE): from MGCF1 to BGCF1
  • If the CS Network is alerting the destination user, MGCF indicates this to the calling party by a 180 Ringing provisional response.
  • As the indication of called party being alerted ("subscriber free" indication) may not be available in ACM, the 180 Ringing is only sent when the indication is available. An ACM without the "subscriber free" indication will not trigger any SIP message.