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VoNR Call Termination Procedure | Voice Over NR Call Release Tutorial

call-flow VoNR | IMS | SIP BYE | RTP

Introduction

The VoNR Call Termination procedure describes how an active voice call is ended in a 5G Standalone network.

A call can be terminated by:

  • the calling UE
  • the called UE
  • the network

When the call is terminated, the IMS network uses SIP signaling to release the session and stop the voice media transmission.

The procedure operates over 5G NR radio access and the IMS core network.

The call termination process is standardized by the 3rd Generation Partnership Project in:

  • 3GPP TS 23.228
  • 3GPP TS 24.229
VoNR call termination procedure diagram
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Network Elements Involved

Several network elements participate in the VoNR call termination procedure.

Network Element Function
UE-ACaller device
UE-BCalled device
gNBNR radio base station
IMS CoreSIP signaling control
UPFUser plane traffic routing

VoNR Call Termination Signaling Flow

UE-A        gNB        IMS        UE-B
 |           |          |          |
 |---SIP BYE---------------------->|
 |           |          |          |
 |<---------200 OK-----------------|
 |           |          |          |
 |===== RTP Media Terminated =====|
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Step-by-Step VoNR Call Termination Procedure

Step 1: Call Release Trigger

The call termination procedure begins when a user decides to end the call.

Typical triggers include:

  • user pressing End Call
  • network session timeout
  • radio link failure

At this point, the UE generates a SIP BYE message.

Important parameters to check

Parameter Description
Call-IDUnique session identifier
From HeaderCalling party identity
To HeaderCalled party identity

Step 2: SIP BYE Sent to IMS

The UE sends a SIP BYE message through the IMS network.

The BYE message indicates that the voice session should be terminated.

The IMS core forwards the message to the other UE participating in the call.

Example SIP BYE Message

BYE sip:user@operator.com SIP/2.0
From: <sip:userA@operator.com>
To: <sip:userB@operator.com>
Call-ID: 123456789

Step 3: SIP 200 OK Response

The receiving UE confirms the termination by sending a 200 OK response.

This response acknowledges the session termination request.

Important parameters to check

Parameter Description
Response Code200 OK
Session IdentifierMatches original session

Step 4: Media Session Termination

Once the SIP exchange is completed:

  • RTP voice media stops
  • IMS releases session resources
  • UPF stops forwarding voice packets

The call is now fully terminated.

RTP Media Release

Voice packets are transmitted using RTP (Real-Time Transport Protocol) during the call.

After termination:

  • RTP stream stops
  • voice session resources are freed

QoS Release

When the call ends, the dedicated QoS flow for voice traffic is released.

Typical QoS parameters include:

Service 5QI
Voice Media5QI = 1
SIP Signaling5QI = 5

This frees network resources for other users.

Troubleshooting Call Termination Issues

Call Not Properly Released

Possible causes:

  • SIP signaling failure
  • network routing issues
  • IMS session mismatch

RTP Stream Continues

Possible reasons:

  • incomplete session teardown
  • IMS signaling failure
  • UPF resource issue

Delayed Call Termination

Possible causes:

  • SIP retransmissions
  • network congestion
  • signaling timeout
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Summary

The VoNR Call Termination procedure ensures that an active voice session is properly released.

The process includes:

  1. UE sending SIP BYE
  2. receiving UE responding 200 OK
  3. RTP media stream stopping
  4. network resources being released

This mechanism ensures efficient session management and proper resource utilization in 5G voice networks.