EPS Fallback: when VoNR isn't available

When VoNR Is Not Available

When VoNR is not supported or unavailable in the current cell, the network uses EPS Fallback to redirect the UE to 4G for VoLTE. This happens when the gNB or AMF indicates VoNR is not supported in the cell, when network policy prefers EPS Fallback in certain coverage areas, or when the UE itself does not support VoNR. EPS Fallback is essential for voice service continuity in early 5G deployments where VoNR coverage is incomplete. The procedure is triggered when the UE initiates a mobile originated voice call or receives a mobile terminated call indication.