Section: 4.3.5.9 Voice domain preference and UE's usage setting
Content: If the UE supports CS fallback, or the UE is configured to support IMS voice, or both, the UE shall include the information element "Voice domain preference and UE's usage setting" in Attach Request, Tracking Area Update Request and Routing Area Update Request messages. The purpose of this information element is to signal to the network the UE's usage setting and voice domain preference for E-UTRAN. The UE's usage setting indicates whether the UE behaves in a voice centric or data centric way (as defined in TS 23.221[ Architectural requirements ]  [27]). The voice domain preference for E-UTRAN indicates whether the UE is configured as CS Voice only, CS Voice preferred and IMS PS Voice as secondary, IMS PS Voice preferred and CS Voice as secondary, or IMS PS Voice only (as defined in TS 23.221[ Architectural requirements ]  [27]). In this Release of the specifications, inter-RAT mobility to/from the NB-IoT RAT is not supported, and GBR bearers are not supported in the NB-IoT RAT. Hence the UE should not include the "Voice domain preference and UE's usage setting" IE when sending an Attach Request or Tracking Area Update Request on the NB-IoT RAT. NOTE: Depending on operator's configuration, the UE's usage setting and voice domain preference for E-UTRAN can be used by the network to choose the RFSP Index in use (see clause 4.3.6). As an example, this enables the enforcement of selective idle mode camping over GERAN/UTRAN for voice centric UEs relying on CS Fallback for voice support in E-UTRAN.