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LTE VoLTE Call Release Procedure Call Flow

call-flow LTE | VoLTE | IMS | SIP

LTE VoLTE call release is the dialog teardown procedure used when an active VoLTE call ends normally or is cleared by one side.

This page focuses on the SIP BYE branch and the LTE cleanup that follows after the voice session is no longer needed.

Introduction

The release path starts while the call is already active. One side sends BYE, the other side returns 200 OK, and the active dialog ends.

After SIP release, the LTE bearer branch may also be cleared if a voice-specific dedicated bearer was in use.

What Is VoLTE Call Release Procedure in Simple Terms?

What starts the procedure: An active VoLTE call is ended by the UE or the remote side.
What the UE and network want to achieve: Tear down the active SIP dialog and remove voice-specific service context cleanly.
What success looks like: BYE receives 200 OK and the call leaves active state cleanly.
What failure means: The dialog does not clear cleanly or the voice-specific bearer or media state remains inconsistent.

Why this procedure matters

Release analysis is important when calls end abnormally, linger in traces, or leave bearer state behind after the user thinks the call is finished.

Quick Fact Sheet

Procedure name	LTE VoLTE Call Release Procedure
Domain	VoLTE dialog teardown
Main trigger	One side ends an active VoLTE call
Start state	VoLTE call is active
End state	The SIP dialog and voice-specific session state are cleared
Main nodes	UE, IMS, LTE bearer context
Main protocols	SIP, LTE bearer cleanup support
Main success outcome	Call clears cleanly and no voice dialog remains active
Main failure outcome	The call is stuck, half-cleared, or leaves voice context behind
Most important messages	BYE, 200 OK
Main specs	TS 24.229

LTE VoLTE Call Release Procedure — Click the diagram to open the full-size in a new tab.

Preconditions

The VoLTE call is already active.
The SIP dialog is still valid and routable.
Any dedicated voice bearer can be cleared after dialog release if needed.

Nodes and Interfaces

Nodes involved

Node	Role in this procedure
UE	Starts or receives the LTE voice or messaging service and exchanges SIP signaling with IMS.
eNB	Carries the LTE radio side used for IMS-capable packet service.
MME / EPC	Preserve LTE access, bearer, and paging continuity behind the IMS transaction.
P-CSCF / S-CSCF	Handle the SIP signaling path used for registration, call control, and service continuity.

Interfaces used

Interface	Path	Role
LTE Uu	UE <-> eNB	Carries the radio access needed before and during IMS service use.
S1-MME	eNB <-> MME	Carries LTE control-plane continuation behind paging, service request, or bearer handling.
Gm	UE <-> P-CSCF	Carries SIP requests and responses between the UE and IMS.

End-to-End Call Flow

UE                 IMS / peer
|--BYE---------------------------->|
|<--200 OK------------------------|
|   voice dialog cleared          |

Major Phases

Phase	What happens
1. Release trigger	One side decides to end the active VoLTE call.
2. BYE transaction	The SIP dialog enters its release exchange.
3. Final confirmation	200 OK confirms the dialog ended cleanly.
4. Cleanup	Voice-specific bearer or service context can now be cleared.

Step-by-Step Breakdown

Step 1: Start call release

Sender -> receiver: UE or peer -> other side

Message(s): BYE

Purpose: Request teardown of the active SIP dialog.

State or context change: The call is now in release progression.

Note: Use BYE as the main release pivot, not the earlier call-setup messages.

Step 2: Confirm release

Sender -> receiver: Peer -> requester

Message(s): 200 OK

Purpose: Confirm that the dialog has been torn down successfully.

State or context change: The SIP call is no longer active.

Note: This is the main success confirmation for the release path.

Step 3: Clear voice-specific context

Sender -> receiver: LTE service context

Message(s): Bearer or media cleanup as needed

Purpose: Remove the dedicated voice-specific service state after the dialog ends.

State or context change: The UE returns to non-call VoLTE-ready state or general IMS-registered state.

Note: Some failures appear only after BYE if the bearer cleanup lingers.

Important Messages

Message	Protocol	Direction	Purpose in this procedure	What to inspect briefly
BYE	SIP	UE or peer -> other side	Starts teardown of the active VoLTE dialog.	Check which side cleared the call and at what time.
200 OK	SIP	Peer -> requester	Confirms that the call release completed cleanly.	Check whether the confirmation arrives quickly and closes the dialog cleanly.
Activate Dedicated EPS Bearer Context Request	NAS	Network -> UE	Useful earlier context when a dedicated voice bearer existed during the call.	Check whether a dedicated voice bearer should also be cleared after release.

Important Parameters to Inspect

Parameter	What it is	Where it appears	Why it matters	Common issues
Release initiator	Which side sent BYE.	BYE	Explains whether the user side or network side cleared the call.	The wrong side is blamed for release.
Dialog identifiers	The call identifiers of the dialog being released.	BYE / 200 OK	Needed to match release to the correct active call.	Release is associated with the wrong call attempt.
Release timing	The exact point where the call moved from active to released.	BYE / 200 OK	Useful when user reports do not match trace timing.	The release looks early or late because the wrong pivot was used.
Dedicated bearer cleanup	Whether a voice-specific bearer remained or was cleared.	Post-release LTE context	Shows whether the LTE side returned to the expected state after release.	The call clears but bearer state lingers.
Final media state	What happened to media after BYE.	Release boundary	Separates normal teardown from abnormal lingering media behavior.	Media seems active after the dialog should have ended.

Successful Completion

Success means the active dialog receives 200 OK to BYE and the voice-specific service state returns to normal non-call status.

Common Failures and Troubleshooting

Symptom	Likely cause	Where to inspect	Relevant message(s)	Relevant interface(s)	Likely next step
BYE is sent but the call does not clear cleanly	The far side or IMS path does not confirm the release.	BYE and the absence or delay of 200 OK.	BYE, 200 OK	Gm	Check whether the BYE reached the far side and whether the dialog stayed routable.
SIP release succeeds but LTE voice context lingers	Bearer cleanup or media-side teardown is incomplete after dialog success.	Post-BYE bearer and service state.	BYE, 200 OK	Gm, LTE bearer path	Move into bearer cleanup analysis after the SIP part is confirmed.
Release timing is disputed	Different traces use different pivot messages to mark call end.	Use BYE and 200 OK as the release boundary.	BYE, 200 OK	Gm	Standardize on the SIP release pair before comparing other traces.

What to Check in Logs and Traces

Use BYE and its final 200 OK as the release boundary.
Check whether the release was initiated by the UE side or the far side.
Look at bearer cleanup only after the SIP release pair is confirmed.

Related sub-procedures

Related message reference pages

Related troubleshooting pages

Notes

A normal call release is simple on the SIP side, but it is still worth checking whether the voice-specific LTE bearer state returned to the expected non-call condition afterward.