Section: 4.3.5.8 IMS voice over PS Session Supported Indication
Content: The serving PLMN shall send an indication toward the UE during the Attach procedure and Tracking Area Update procedures if an IMS voice over PS session is supported. The serving PLMN uses this indicator to indicate to the UE whether it can expect a successful IMS voice over PS session according to TS 22.173[ IP Multimedia Core Network Subsystem (IMS) Multimedia Telephony Service and supplementary services; Stage 1 ]  [73] with a bearer that supports Conversational Voice as specified in TS 23.203[ Policy and charging control architecture ]  [6]. A UE with "IMS voice over PS" voice capability should take this indication into account when establishing voice over PS sessions (as specified in TS 23.221[ Architectural requirements ]  [27]) as well as when determining whether to deactivate the special handling of ISR locally (as detailed in clause 4.3.5.6). The serving PLMN provides this indication based e.g. on local policy, HPLMN, Voice Support Match Indicator, the SRVCC capability of the network and UE and/or extends of E-UTRAN/UTRAN coverage. The serving PLMN shall indicate to the UE that the UE can expect a successful IMS voice over PS session only if the MME is configured to know that the serving PLMN has a roaming agreement for IMS voice with the HPLMN of the UE. This indication is per TAI list. On request by the HSS, the MME shall indicate the following: - whether or not an IMS voice over PS Session is supported in the TA(s) that are registered for the UE ("IMS voice over PS Session Supported Indication"), together with the time of the last radio contact with the UE; and - the current RAT type. NOTE: In order to support routing of incoming IMS voice calls to the correct domain (PS or CS), the network-based T-ADS (see TS 23.292[ IP Multimedia Subsystem (IMS) centralized services; Stage 2 ]  [60] and TS 23.221[ Architectural requirements ]  [27]) requires that there is homogeneous support/non-support of IMS voice over PS session for all registered TAs of the UE.