5.16.3.2 IMS voice over PS Session Supported Indication over 3GPP access

The serving PLMN AMF shall send an indication toward the UE during the Registration procedure over 3GPP access to indicate if an IMS voice over PS session is supported or not supported in 3GPP access and non-3GPP access. A UE with "IMS voice over PS" voice capability over 3GPP access should take this indication into account when performing voice domain selection, as described in clause 5.16.3.5.

The serving PLMN AMF may only indicate IMS voice over PS session supported over 3GPP access in one of the following cases:

• If the network and the UE are able to support IMS voice over PS session in the current Registration Area with a 5G QoS Flow that supports voice as specified in clause 5.7.
• If the network or the UE are not able to support IMS voice over PS session over NR connected to 5GC, but is able for one of the following:
• If the network and the UE are able to support IMS voice over PS session over E-UTRA connected to 5GC and the NG-RAN supports a handover or redirection to E-UTRA connected to 5GC for this UE at QoS Flow establishment for IMS voice;
• If the UE supports handover to EPS, the EPS supports IMS voice and the NG-RAN supports a handover to EPS for this UE at QoS Flow establishment for IMS voice; or
• If the UE supports redirection to EPS, the EPS supports IMS voice and the NG-RAN supports redirection to EPS for this UE at QoS Flow establishment for IMS voice.
• If the network is not able to provide a successful IMS voice over PS session over E-UTRA connected to 5GC, but is able for one of the following:
• If the UE supports handover to EPS, the EPS supports IMS voice and the NG-RAN supports a handover to EPS for this UE at QoS Flow establishment for IMS voice; or
• If the UE supports redirection to EPS, the EPS supports IMS voice and the NG-RAN supports redirection to EPS for this UE at QoS Flow establishment for IMS voice.

The serving PLMN provides this indication based e.g. on local policy, UE capabilities, HPLMN, whether IP address preservation is possible, whether NG-RAN to UTRAN SRVCC is supported and how extended NG-RAN coverage is and the Voice Support Match Indicator from the NG-RAN (see clause 4.2.8a of TS 23.502 [3]).

NOTE 1: The terms "UE supports handover to EPS" or "UE supports redirection to EPS" used above also consider the case that the UE has signalled that S1 mode is enabled. In case the UE has signalled that S1 mode is disabled for a network that only supports IMS voice via EPS Fallback, the AMF will not indicate that IMS voice over PS session is supported over 3GPP access. A voice centric UE ensures its voice service as described in clauses 5.16.3.5 and 5.16.3.6.

The AMF in serving PLMN shall indicate that IMS voice over PS is supported only if the serving PLMN has a roaming agreement that covers support of IMS voice with the HPLMN. This indication is per Registration Area.

NOTE 2: If the network supports EPS fallback for voice the 5GC can be configured not to perform the Voice Support Match Indicator procedure in order to set the IMS voice over PS session Supported Indication.

The serving SNPN provides the IMS voice over PS indication based e.g. on local policy, UE capabilities, whether IP address preservation is possible and how extended NR coverage is. This indication is per Registration Area.

NOTE 3: Since over 3GPP access, in SNPN access mode there is only support for NR and the "voice centric" UE cannot reselect to another RAT in the same registered SNPN if the first Registration Area that the UE tries to register from cannot support IMS voice, it is recommended that support for IMS voice is provided homogeneously in the whole SNPN if at all.