Although the demand for data services is the main driving force for the growth and evolution of 5G, voice services remain an indispensable part of mobile communication networks.

The new radio voice (VoNR) function provided by 5G is similar to VoLTE in 4G. VoNR is a voice solution based on IMS under the 5G SA architecture. In VoNR, a UE resides on a 5G network, and voice and data services are provided through gNB and 5GC on the 5G network. When the UE is not within the coverage of NR, the inter-RAT handover will be initiated to provide VoLTE within the LTE coverage.

In the early stage of 5G, if the VoNR function has not yet been deployed on the SA architecture network, or if the network has deployed the NSA Option 3 architecture but lacks 5GC, the network will not support the VoNR function. In NSA mode, since the network has EPC, VoLTE is used to provide voice services of LTE, while data services are provided by NR. However, in SA mode, the network does not support VoLTE.

Therefore, in order to provide continuous voice services when VoLTE calls move to 5G SA networks, 3GPP defines a voice fallback function. 5G has two types of FallBack functions: Evolved Packet System (EPS) fallback and RAT fallback.

• In EPS fallback, if the UE attempts to use voice services in a 5G network that does not support VoNR, the gNB will redirect or switch to the LTE network, so the UE can use voice services via VoLTE. After the voice session is terminated, the UE can switch back to the 5G network.

• RAT fallback is similar to EPS fallback, except that the UE falls back to the 5GC-controlled eLTE network during a voice session.
  

VoNR is only applicable to NR deployments equipped with 5GC and gNB. Therefore, operators need to utilize VoLTE services and maintain EPCs to provide continuous voice services until VoNR is available. As an alternative to 5G voice solutions in regions that do not support VoLTE and VoNR, 5G-SRVCC can be provided through the UMTS Ground Radio Access Network (UTRAN) to offer continuous voice services.

Among them, SRVCC (Single Radio Voice Call Continuity) is a VoLTE voice service continuity solution proposed by 3GPP, mainly to solve the problem when a single radio frequency UE moves between LTE/Pre-LTE networks and 2G/3G CS networks, The issue of how to ensure the continuity of voice calls, that is, to guarantee the smooth switching between IMS controlled VoIP voice and CS domain voice with a single radio frequency UE.