3GPP 23.501 v20.2.0 — the document's own text

5.16.3.3 Homogeneous support for IMS voice over PS Session supported indication

Taught in 21. Emergency calls, messages, voice and location (The 5G system architecture, in depth).

5GC shall support the usage of "Homogeneous Support of IMS Voice over PS Sessions" indication between AMF and UDM.

When the AMF initiates Nudm_UECM_Registration operation to the UDM, it shall:

if "IMS Voice over PS Sessions" is supported homogeneously in all TAs in the serving AMF for the UE, include the "Homogeneous Support of IMS Voice over PS Sessions" indication set to "Supported";
if none of the TAs of the serving AMF supports "IMS Voice over PS Sessions" for the UE, include the "Homogeneous Support of IMS Voice over PS Sessions" indication set to "Not supported";
if "IMS Voice over PS Sessions" support is either non-homogeneous or unknown, not include the "Homogeneous Support of IMS Voice over PS Sessions" indication.

The AMF shall be able to provide the "Homogeneous Support of IMS Voice over PS Sessions" indication as described above to the UDM using Nudm_UECM_Update operation as specified in clause 4.2.2.2.2 of TS 23.502 [3].

The UDM shall take this indication into account when doing Terminating Access Domain Selection (T-ADS) procedure for IMS voice.

NOTE: A TA supports "IMS Voice over PS Sessions" if the serving AMF indicates IMS voice over PS Session Supported Indication over 3GPP access to the UE, as described in clause 5.16.3.2. In order to support routing of incoming IMS voice calls to the correct domain, the network-based T-ADS (see TS 23.292 [63] and TS 23.221 [23]) requires that the "Homogeneous Support of IMS Voice over PS Sessions" indication is set to "Supported" for all registered TAs of the UE or "Not supported" for all registered TAs of the UE.