IP Multimedia Subsystem (IMS) provides the foundation for voice across 5G system.
To provide voice as a 5G application service, The IMS network will typically use GSMA NG.114 as the IMS profile
There are two solutions to provide Voice Over 5G (Vo5G):
VoNR: gNB and 5GC provide the resources (Dedicated QoS Flow) to carry VoNR.
EPS Fallback or RAT Fallback:
a) A PDU Session contains a default QoS Flow is created toward the IMS network to exchange the SIP.
b) The PDU SESSION RESOURCE MODIFY REQUEST message to setup a QoS flow for IMS voice is sent down to the gNB by AMF via NGAP.
c) The 5G gNB and NR cell doesn’t provide voice services (e.g. 5G NR cell is not configured or UE doesn’t have all needed VoNR capability).
d) The gNB initiates EPS fallback or RAT fallback for IMS voice procedure and report unsuccessful establishment of the QoS flow to the AMF using NGAP with cause value "IMS voice EPS fallback or RAT fallback triggered“.
e) Based upon the scenario and 5G network configuration, Release with Redirect (RwR) to LTE or EPS fall back is selected by 5GS to send the UE to the 4G cell for the VoLTE call.
Here I try to depict a high level architecture of VoNR based on 3GPP Rel.16: