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EPS Fallback Procedure Call Flow

call-flow 5G SA | LTE | EPS Fallback | VoLTE | IMS

EPS Fallback is the voice continuity procedure that moves a UE from 5G SA to LTE/EPS when voice service cannot remain on native NR.

The real target is not only a successful inter-RAT move, but a usable voice session on LTE through VoLTE / IMS.

Introduction

This page explains EPS fallback as a complete end-to-end branch: voice trigger on NR, fallback decision, move to LTE, LTE/EPS readiness, and final SIP call continuity.

In troubleshooting, always separate why fallback started, whether the UE reached LTE, and whether voice actually continued after LTE landing.

What Is EPS Fallback Procedure in Simple Terms?

What starts the procedure: A voice event appears on 5G SA and native NR voice is not the right path.
What the UE and network want to achieve: Move the UE to LTE quickly enough for stable VoLTE / IMS voice continuity.
What success looks like: The UE reaches LTE and the voice call continues successfully.
What failure means: Fallback starts late, mobility fails, or LTE / IMS voice continuity does not come up.

Why this procedure matters

EPS fallback issues are highly visible to users because they affect live call setup and voice continuity. The wrong diagnosis can easily send engineers to the wrong layer.

Quick Fact Sheet

Procedure name	EPS Fallback Procedure
Domain	5G SA voice continuity fallback from NR to LTE/EPS
Main trigger	Voice service cannot stay on native NR and must continue through LTE/VoLTE
Start state	UE is active on 5G SA and a voice event requires LTE/EPS fallback handling
End state	UE reaches LTE and the voice service continues through VoLTE / IMS on EPS
Main nodes	UE, gNB, AMF, eNB, MME/EPC, IMS core
Main protocols	NR mobility signaling, inter-RAT fallback handling, LTE access, IMS / SIP continuity
Main success outcome	The UE leaves NR, reaches LTE, and the voice call continues through VoLTE successfully
Main failure outcome	Fallback is late, mobility fails, or LTE / IMS voice continuity is not established
Most important messages	Voice trigger, fallback decision, NR release or inter-RAT move, LTE access, IMS SIP call setup
Main specs	TS 23.502, TS 23.501, TS 38.300, TS 38.331, TS 23.401, TS 24.229

Preconditions

The UE is already on 5G SA with a voice-related event in progress or about to begin.
The network has a valid reason to prefer LTE/EPS for voice continuity.
An LTE target and EPS voice environment are available.
IMS / VoLTE service can be used after the UE reaches LTE.

Nodes and Interfaces

Nodes involved

Node	Role in this procedure
UE	Starts or receives the voice event and executes the move from NR toward LTE/EPS.
gNB	Serves the UE on NR before fallback and participates in the release or inter-RAT move.
AMF	Controls the 5G-side mobility and fallback decision path.
eNB	Becomes the serving radio node after fallback to LTE.
MME / EPC	Provide the EPS mobility and bearer environment needed once the UE lands on LTE.
IMS core	Carries the VoLTE SIP call setup or continuation after fallback completes.

Interfaces used

Interface	Path	Role
NR-Uu	UE <-> gNB	Carries the voice-trigger context and the last NR-side mobility branch.
LTE-Uu	UE <-> eNB	Carries the LTE-side access and later VoLTE radio service.
N2	gNB <-> AMF	Supports fallback decision and NR-side coordination.
Inter-RAT mobility interfaces	5G side <-> LTE/EPS side	Coordinate the move toward LTE and bearer continuity.
IMS / SIP signaling path	UE <-> IMS core over LTE	Carries the actual voice session once fallback succeeds.

End-to-End Call Flow

UE                gNB / AMF              eNB / EPC              IMS
|                     |                        |                    |
|-- voice trigger --->|                        |                    |
|                     |-- fallback decision -->|                    |
|<==== NR to LTE move / redirection / HO =========================>|
|                     |                        |-- voice ready ---->|
|================ VoLTE / IMS call continuity on LTE ============>|

Major Phases

Phase	What happens
1. Voice trigger on NR	A mobile-originated, mobile-terminated, or emergency voice event appears while the UE is on 5G SA.
2. Fallback decision	The network decides that voice should continue through LTE/EPS instead of native NR voice.
3. Move to LTE	The UE is redirected or handed over from NR toward LTE.
4. LTE voice readiness	The UE becomes active on LTE and the EPS / IMS environment is ready for voice service.
5. VoLTE / IMS call continuity	The voice call proceeds through LTE and IMS after fallback.

Step-by-Step Breakdown

A voice event starts while the UE is on 5G SA

Sender -> receiver: UE <-> AMF / IMS context

Message(s): Voice trigger, paging, or service-originating event

Purpose: Start the voice branch that will either stay on NR or fall back to LTE.

State or context change: The network must decide whether native NR voice is usable for this case.

Note: Always classify whether the case is MO, MT, or emergency voice before reading the fallback branch.

The network decides EPS fallback is required

Sender -> receiver: AMF / gNB policy and capability logic

Message(s): Fallback decision and mobility preparation

Purpose: Choose LTE/EPS as the voice-serving environment.

State or context change: The voice branch becomes a specific inter-RAT mobility case with voice continuity intent.

Note: Fallback can be driven by VoNR support, policy, roaming, coverage, or service constraints.

The UE leaves NR and moves to LTE

Sender -> receiver: gNB -> UE -> eNB

Message(s): NR release, redirection, or inter-RAT handover toward LTE

Purpose: Move the UE to the RAT that will actually carry the voice service.

State or context change: The UE transitions from NR service toward LTE/EPS reachability.

Note: Many EPS fallback failures are actually mobility failures before SIP even starts.

LTE and EPS voice context become ready

Sender -> receiver: UE <-> eNB <-> EPC

Message(s): LTE access and EPS service continuity

Purpose: Provide the bearer and access environment needed for VoLTE / IMS.

State or context change: The UE is now positioned to continue the voice call over LTE.

Note: Do not treat LTE landing as equivalent to voice success. IMS still must work afterward.

VoLTE / IMS signaling confirms voice continuity

Sender -> receiver: UE <-> IMS core over LTE

Message(s): SIP INVITE and related IMS call-control messages

Purpose: Prove the fallback achieved its real goal: usable voice service.

State or context change: The EPS fallback branch ends in voice continuity on LTE.

Note: The final proof is SIP and voice continuity, not just the radio move to LTE.

Important Messages in This Flow

Message	Protocol	Direction	Purpose in this procedure	What to inspect briefly
Voice trigger context	Service / NAS	UE or network	Explains why the fallback branch started.	Classify whether the case was MO, MT, or emergency voice.
Inter-RAT move toward LTE	RRC / mobility	NR side -> UE -> LTE side	Actually moves the UE off NR and onto LTE.	This is where many fallback failures begin.
LTE access and EPS readiness	LTE / EPC control	UE <-> LTE/EPS	Provide the bearer environment needed for voice over LTE.	Needed before IMS can really help.
IMS SIP call setup	SIP	UE <-> IMS core	Proves the fallback achieved actual voice continuity.	Final voice-success checkpoint.

Important Parameters to Inspect

Parameter	What it is	Where it appears	Why it matters	Common issues
Voice trigger type	Why voice service started on NR.	Before fallback decision	Explains the specific service branch the network is handling.	Important for correct troubleshooting context.
VoNR support and policy state	Why fallback was allowed or required.	Fallback decision stage	Separates capability-driven fallback from policy-driven fallback.	This is often the first root-cause checkpoint.
NR-to-LTE mobility outcome	Whether the UE really reached LTE cleanly.	Inter-RAT move stage	Primary split between mobility failure and later voice failure.	Do not skip this distinction.
LTE voice readiness	Whether the LTE / EPS environment is prepared for VoLTE.	Post-move stage	Needed before SIP call continuity can succeed.	Many failures hide here.
SIP call outcome	Actual result of the voice service after fallback.	Final validation stage	Best proof that EPS fallback delivered the expected service.	This is the user-visible truth source.

Success Criteria

The fallback decision is taken for the right capability, policy, or coverage reason.
The UE moves from NR to LTE cleanly enough for voice continuity.
LTE / EPS voice context is ready after landing.
IMS SIP signaling confirms actual voice continuity on LTE.

Common Failures and Troubleshooting

Symptom	Likely cause	Where to inspect	Relevant message(s)	Relevant interface(s)	Likely next step
Fallback is triggered too late	The network waits too long before moving the UE to LTE for voice.	Trigger timing and early call-control timing.	Decision stage	Cross-layer timing	Usually shows up as long setup delay or call failure.
NR-to-LTE move fails	The UE cannot complete the inter-RAT mobility needed for fallback.	Fallback move and LTE access behavior.	Inter-RAT move stage	Radio / mobility	If this fails, IMS often never gets a fair chance.
LTE landing succeeds but voice still fails	The UE reached LTE, but VoLTE / IMS continuity is not ready or aligned.	LTE/EPS readiness and SIP outcome.	Post-move and SIP stage	LTE / IMS	This is a voice continuity failure after mobility success.
Capability or policy logic is wrong	The network chose a voice path that does not match the device or service policy.	Capability, subscription, and policy handling.	Decision stage	5G core / subscriber logic	Often misread as radio trouble later.

What to Check in Logs and Traces

Classify the voice trigger type before reading the fallback trace.
Inspect VoNR support and policy to understand why fallback was chosen.
Separate NR-to-LTE mobility success from later voice success.
Check LTE / EPS readiness before blaming IMS alone.
Use SIP outcome as the final proof of successful fallback.

Related sub-procedures

Related message reference pages

Related troubleshooting pages

FAQ

What is EPS Fallback?

It is the procedure that moves a 5G SA UE to LTE/EPS so voice can continue through VoLTE when native NR voice cannot be used.

Is it just generic inter-RAT handover?

No. It is a specific inter-RAT mobility branch whose goal is voice continuity through LTE/IMS.

What proves success?

The UE reaches LTE and the voice call continues successfully through VoLTE / IMS.

What should I inspect first?

Start with why fallback was triggered, then the NR-to-LTE move, then LTE / IMS voice continuity.

Why can the call still fail after reaching LTE?

Because mobility success does not automatically mean EPS readiness, IMS readiness, or correct SIP continuity.