The C-MSISDN, Correlation Mobile Station ISDN number, is one of the variables that is used to identify an anchored voice session in IMS. Below is a list of events that take place to clarify the use of the C-MSISDN during a PS-to-CS handover for a VoLTE session:

  1. The SCC AS is the anchor of the call in the IMS domain.
  2. Per TS 23.216, during the Attach process, the HSS sends the C-MSISDN to the MME as part of the UE’s subscription data.
  3. Per TS 24.237, the SCC AS obtains registration state information from the HSS which includes the C-MSISDN that is bound to the IMS private user id per TS 23.237.
  4. Per TS 23.237, the SCC AS provides the C-MSISDN to the ATCF upon successful IMS registration.
  5. As the UE moves out of LTE coverage, the MME requests the MSC to transfer the call to legacy technologies such as 3G.
  6. Per TS 23.216, the MME sends the C-MSISDN to the MSC as part of the SRVCC PS to CS handover.
  7. Per TS 23.856, “The MSC Server correlates the incoming PS to CS Handover request with the anchored session using the C-MSISDN obtained when anchoring the session”.
  8. MSC sends an INVITE to the IMS side which includes the C-MSISDN (as a P-Asserted-Identity) representing the calling party number.
  9. Upon receiving the INVITE, the SCC AS determines that an access transfer is being requested. The SCC AS uses the STN-SR and the C-MSISDN to identify the anchored session.