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For a VoLTE only capable E‐UTRAN UE and LTE/EPC with IMS capability, the UE will send the following information during its initial registration, through the ATTACH Request message, procedure with the EPC:

 UE to EPC (ATTACH Request)UE Usage Setting = Data‐CentricVoice domain preference = IMS Voice Only

The LTE/EPC will send the following information as part of the ATTACH ACCEPT message to the UE:

 EPC to UE (ATTACH Accept)EPS Attach Result = EPS OnlyEPS network feature support = IMS voice‐over PS session in S1 mode supported

For more details on the UE mode of operations, its usages, voice domain preference, and the tasks performed in each transition, refer to TS 24.301 [46].