Apple Inc.
5G NR Voice Call EPS Fallback Enhancements

Abstract:

Apparatuses, systems, and methods for a user equipment device (UE) to perform methods for EPS fallback when initiating a voice call while camped on a 5G cell. The UE may camp on a 5G network by performing an IMS registration procedure/IMS SIP invite procedure with an IMS. The UE may initiate a voice call and determine that the voice call cannot be established over the 5G NR network. In response, the UE may perform a TAU procedure or an attachment procedure with a cell of a 4G network. The IMS may be transferred to the 4G network during TAU/attachment procedure. The UE may, in response to determining that it is barred from the 4G network, transition back to the 5G network and transmit, to the 5G NR network, a SIP cancel request to allow a context associated with the UE to be cleared from the IMS.