23.1673GPPIP Multimedia Subsystem (IMS) emergency sessionsRelease 17TS
This annex includes network impact and call flows for support of IMS emergency sessions for roaming users in deployments without IMS-level roaming interface between the P-CSCF in the VPLMN and the S-CSCF in the HPLMNs. This annex also applies to IMS emergency sessions for SNPN users when there is no support for IMS-level roaming, or the user has no IMS credentials. This annex is only applicable to UTRAN, E-UTRAN and NG-RAN access networks.
IMS authentication is performed by linking the IMS registration to user’s EPS or 5GS authentication based on the user’s IP address and IMSI. This mechanism is similar to GIBA (GPRS-IMS bundled authentication) specified in TS 33.203 . The P-CSCF performs the GIBA procedure over Gm with the UE as defined in TS 24.229 . If this procedure is not supported, UE may perform anonymous emergency session. A callback number for the user may also be retrieved via the PCRF or PCF.