EPS Fallback
A mechanism where a VoNR call request triggers the UE to fall back from 5G SA to 4G/LTE to use VoLTE when IMS over NR is not available in the serving area.
EPS Fallback is the safety valve that let operators launch 5G data long before voice over 5G was ready everywhere. The phone camps on 5G SA and uses it happily for data. The moment a voice call starts — the IMS INVITE goes out — and the serving cell can't do VoNR, the network moves the device to LTE (by redirection or inter-system handover) and the call completes there as VoLTE. To the user it's just a normal call, perhaps with a slightly longer setup time.
Mechanically, the trigger is the dedicated QoS flow request for voice (5QI 1) arriving at a gNB that doesn't support it; the gNB then initiates the fallback. The trade-off is that setup latency: the inter-system move adds time before the phone rings, and a handover-based fallback is smoother than a redirection-based one. It's meant as a bridge, not a destination — as VoNR coverage fills in, fallback events in that area taper off.
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