As the VoLTE adoption cycle, it took the industry a few years to plan, test, and commercialize new voice services, and I believe VoNR will follow a similar timeline. And since 4G is the foundation for 5G NR deployments, I expect VoLTE to remain the 5G voice solution for many years to come.
5G NSA
In initial 5G deployments using option 3, 5G EPS includes NR nonstandalone in the RAN. 4G voice (VoLTE) and other IMS services continue to be used without the need for any core network upgrade. IMS is not aware of the updated RAN. All 4G VoLTE principles remain.
5G SA
For 5G standalone NR deployment, SA2 defined VoNR (Voice over NR) and VoNR -> VoLTE fallback as voice solution in R15.
5G system (5GS) using option 2 includes NR standalone in the RAN. At least initially, a 5G System (5GS) will not be deployed with the full network coverage. Therefore, the 5GS needs to be tightly coupled to an existing 4G VoLTE deployment, to provide seamless voice services across the whole 4G+5G network with good performance.
EPC Fallback
EPS fallback is used in early 5GS deployments; the UE falls back from 5G RAN to LTE during call establishment. Hence all voice-related capabilities are needed in 5G RAN, 5GC, and IMS except those needed to completely
establish the call over NG-RAN.
Voice over NR means voice calls can be made on 5G New Radio. This requires NR in 5G to support all voice capabilities, being dimensioned for voice coverage. Handover of voice calls between 5GS and 5G EPS can be performed, based on measurement reports provided by the UE.
5G Will Take 3 Stages for Voice Solutions:
- 5G NSA phase: When 5G services are launched, operators should, by default, enable VoLTE services for 5G NSA subscribers.
- 5G SA initial phase: As defined in 3GPP R-15, CS fallback from 5G to 2G/3G is not supported. This means that during the voice call establishment phase, 5G SA networks should use Evolved Packet System Fallback (EPS FB) technology for voice fallback to the LTE network, completing the connection through VoLTE.
- 5G SA mature phase: Terminal support for 5G SA will need to become mainstream, and their default configuration will need to be for VoNR (set to ON by default). When the user moves outside of 5G coverage during a call, Packet Switched Handover (PSHO) technology can be used to seamlessly switch the session from the NR to the LTE, leaving VoLTE to take over voice services. (This is like SRVCC in 4G, but the principles are different).