EPS Fallback Procedure | 5G to LTE Voice Fallback Call Flow

Introduction

EPS Fallback is the procedure used when a UE is attached to a 5G Standalone network but the voice call cannot stay on native VoNR.

Instead of continuing the call on NR, the network moves the UE to LTE/EPS so the voice session can be handled using VoLTE and IMS.

This page is most useful when read together with VoNR Registration, VoNR Mobile Originated Call, VoNR Mobile Terminated Call, and 5G Inter-RAT Handover.

The fallback behavior is defined across the 5G system procedures, NR RRC / NG-RAN mobility behavior, EPC interworking, and IMS voice handling specifications.

Engineers usually search this procedure to answer questions such as:

• why did a 5G voice call move to LTE?
• was fallback triggered by missing VoNR support or by policy?
• did the redirection/handover complete before SIP call setup?
• where should I look in NR, LTE, and IMS traces?

Key Specifications for EPS Fallback

EPS fallback is not described by a single message-level spec. In practice, engineers need to correlate the following 3GPP references together.

When EPS Fallback Is Used

EPS fallback is normally used when VoNR is not available or not allowed, but the subscriber still needs voice service.

Network Elements Involved

EPS Fallback Call Flow Position

UE          gNB/AMF           eNB/MME/EPC           IMS
 |              |                  |                 |
 |-- Voice trigger on NR --------->|                 |
 |              |-- Fallback decision -------------> |
 |<-- Redirect / HO to LTE ------- |                 |
 |============= LTE access / continuity ============>|
 |---------------- SIP INVITE / VoLTE ------------->|
 |<--------------- 100 Trying / 180 / 200 ----------|

In most practical traces, the engineer must correlate the last NR-side control messages, the inter-RAT mobility step, and the first VoLTE-side IMS signaling.

Step-by-Step EPS Fallback Procedure

Step 1: UE Is Attached on 5G SA

The UE is already registered on the 5G network, often after completing 5G Initial Registration.

Depending on deployment, the UE may also have completed VoNR / IMS registration checks, or the network may know upfront that voice must use EPS fallback.

Step 2: Voice Trigger Arrives

A voice event occurs, usually either:

• a mobile originated call attempt
• a mobile terminated paging / incoming call
• an emergency call trigger

At this point the network decides whether the call can stay on NR or should move to LTE for VoLTE handling.

Step 3: Network Triggers Fallback

The AMF / NG-RAN applies the fallback decision based on policy, device capability, network support, or coverage state.

The UE is then redirected or handed over toward LTE.

Step 4: UE Moves to LTE

The UE leaves NR and acquires LTE, either through redirection or inter-RAT handover behavior that is deployment-specific.

This is the part that should be compared with 5G Inter-RAT Handover and, if needed, the LTE access procedure set.

Step 5: VoLTE / IMS Voice Setup Continues

Once LTE access is available, the voice call continues using the LTE/EPS path and standard IMS signaling. For the LTE-side voice sequence, compare this with the live VoLTE call flow and procedures reference.

Typical IMS procedures that become relevant next are:

• mobile originated call setup for comparison with native VoNR
• mobile terminated call setup for incoming-call behavior
• call release and session cleanup

Key Signals and Interfaces to Correlate

What to Check in Logs and Traces

• Whether the UE was VoNR capable and VoNR allowed by policy.
• Whether the trigger was an MO call, MT call, or emergency scenario.
• The exact NR message or decision point that started fallback.
• Whether LTE acquisition or inter-RAT movement completed successfully.
• Whether IMS signaling resumed correctly on the LTE side.
• Whether the call failed before fallback, during mobility, or after LTE access.

Common EPS Fallback Failure Points

Fallback Triggered Too Late

If the decision happens late, the user may see long call setup time or call setup failure.

NR to LTE Mobility Failure

If the UE cannot complete the move to LTE, the call usually fails before IMS can establish the VoLTE session.

IMS / VoLTE Not Ready After LTE Move

The radio fallback may succeed, but the voice call can still fail if the IMS path, LTE bearer setup, or subscriber profile is not aligned.

Capability or Policy Mismatch

The network may attempt a voice path that does not match the UE capability, roaming case, or configured service policy.

SEO and Engineering Search Terms This Page Should Capture

Summary

EPS Fallback is the practical bridge between 5G SA data service and LTE-based voice service when native VoNR is not available.

In real troubleshooting, the key is to correlate three domains together:

1. the NR-side trigger
2. the NR to LTE movement
3. the VoLTE / IMS call setup after fallback

For deeper analysis, continue with VoNR Call Flow Procedures, 5G Inter-RAT Handover, 5G RRC Messages, and the 3GPP Decoder.

Recommended Reference Specifications

• 3GPP TS 23.502 - 5G System Procedures
• 3GPP TS 23.501 - System Architecture for the 5G System
• 3GPP TS 38.300 - NR and NG-RAN Overall Description
• 3GPP TS 38.331 - NR RRC Protocol
• 3GPP TS 36.331 - E-UTRA RRC Protocol
• 3GPP TS 23.401 - GPRS Enhancements for E-UTRAN Access
• 3GPP TS 24.229 - IMS SIP Signalling