ds to seamlessly hand over a VoNR call to VoLTE. To achieve this, a Packet Switched (PS) handover must be coordinated Interworking and voice call continuity between the 5GC and 4G core network Evolved Package System (EPS), which requires interworking between the AMF of 5G and the Mobility Management Entity (MME) of 4G. This interworking is coordinated by the N26 interface between the AMF and the MME; hence, it needs to be provisioned in the network. Single radio voice call continuity There might be scenarios where 5G network coverage is diminishing and where there is no 4G network available. In this case, for voice call continuity, Single Radio Voice Call Continuity (SRVCC) can be employed assuming the presence of 3G or 2G networks. SRVCC allows a PS-to-CS domain switch of an ongoing IMS call. SRVCC has been in use for many years for 4G networks to switch the VoLTE call to 3G or 2G networks where 4G coverage or quality diminishes. Note that 3GPP Release 15 doesn't cover SRVCC. SRVCC for 5G towards 3G is available in 3GPP Release 16. The SRVCC procedure is coordinated through the AMF. The AMF uses the MME in the EPS as a bridge to communicate with the Mobile Switching Center - Server (MSC-S) placed in the 3G core network. The MSC-S is a core network node to control voice services. When the handover decision is made by gNB, the AMF sends a handover request to the MSC-S via the MME. When the MSC-S receives the handover request, it coordinates the 3G network to set up radio resources for the voice call. After that, the MSC-S sends a redirect request to the SCC or to the Access Transfer Control Function (ATCF) in the IMS, depending on the IMS deployment. So, this redirect request basically takes the VoNR call away from the 5G packet switched network onto the 3G circuit switch core network: Handover Request MSC-S Redirect Request Figure 8.12 - The SRVCC procedure Voice over New Radio (VoNR) EPS fallback Service providers may choose not to carry conversational voice with VoNR in the early 5G standalone (SA) deployments,