VoNR Call Hold / Resume | 5G Voice Over NR Call Flow Tutorial
Introduction
VoNR Call Hold / Resume is the 5G IMS procedure used to temporarily suspend and later continue the media of an active voice session without releasing the call itself.
In 3GPP terms, VoNR is IMS-based communication service over 5G NR, and IMS continues to use SIP for session signaling and SDP for media description.
3GPP also maintains dedicated IMS and MMTel specifications for Communication HOLD and common communication procedures, which is why hold and resume is mainly an IMS service-layer procedure rather than a brand-new NR-specific call setup.
VoNR Call Hold / Resume Introduction
When a user presses Hold during a VoNR call, the call is not disconnected. The SIP dialog remains established, but the media direction is changed so that voice packets are paused in one or both directions.
When the user presses Resume, the same session is updated again and normal media flow continues.
In practical deployments, this means VoNR hold and resume behaves much more like a session modification than a fresh call establishment.
The UE, P-CSCF, S-CSCF, and far-end UE remain part of the active dialog, and depending on supplementary-service handling, TAS and MRF can also be relevant in the end-to-end service path.
Network Elements Involved
For a normal VoNR hold and resume flow, the main nodes are:
- UE: The user presses Hold or Resume, and the UE generates the in-dialog SIP session modification.
- P-CSCF: First IMS contact point for the UE, forwarding SIP signaling.
- S-CSCF: Main IMS session-control node for the subscriber.
- TAS: May participate when operator supplementary-service logic is applied.
- MRF: May provide tones or announcements such as music-on-hold, depending on implementation.
- Far-end UE or interworking node: Receives the modified offer and updates media behavior accordingly.
How Hold Is Signaled in VoNR
At SIP and SDP level, hold is typically indicated by sending a new SDP offer inside the existing dialog.
Hold is commonly signaled by a new offer with a=sendonly for the media stream being held, and the answerer acknowledges it with a=recvonly.
If both sides end up wanting no media in either direction, a=inactive can appear instead.
The key point is this: the call remains established, but the media direction changes.
So, when troubleshooting VoNR hold, you usually inspect the re-INVITE, 200 OK, and ACK exchange and the SDP direction attributes before looking at deeper radio issues.
VoNR Call Hold Procedure
A simple mobile-to-mobile VoNR hold flow looks like this:
- User on UE-A presses Hold.
- UE-A stops normal media transmission and sends an in-dialog re-INVITE with SDP indicating hold, typically
a=sendonly. - The request is forwarded through P-CSCF / S-CSCF and reaches the far-end side.
- UE-B accepts the change and returns 200 OK with SDP, commonly
a=recvonlyora=inactivedepending on the negotiated state. - The response travels back through IMS to UE-A.
- ACK completes the SIP transaction, and the call remains active but held.
Hold Call Flow Diagram
UE-A P-CSCF S-CSCF/TAS Remote IMS UE-B
| | | | |
|--re-INVITE (SDP: a=sendonly)-------------------->|
| | |------------->| |
| |------------->| |
| | | |--re-INVITE (hold request)-->|
| | | | |
| | | |<--200 OK (SDP: a=recvonly / a=inactive)--|
|<-------------------------------200 OK-------------------------------|
|-------------ACK---------------------------------------------------->|
| | | | |
|====== Call stays established, media is on hold ====================|The exact SIP method set may vary by implementation details, but the standards-based model is consistent: session modification inside the existing IMS dialog, not a new mobile-originated call setup.
VoNR Call Resume Procedure
Resume reverses the hold state. The endpoint that placed the media on hold later sends the resume message, the network forwards it through the same IMS path, and media starts flowing again after successful completion.
In SIP and SDP terms, resume usually means sending a fresh SDP offer that restores normal media direction, commonly a=sendrecv.
Once the far end replies with 200 OK and the transaction is acknowledged, RTP media continues in both directions again.
Resume Call Flow Diagram
UE-A P-CSCF S-CSCF/TAS Remote IMS UE-B
| | | | |
|--re-INVITE (SDP: a=sendrecv)--------------------->|
| | |------------->| |
| |------------->| |
| | | |--re-INVITE (resume request)-->|
| | | | |
| | | |<--200 OK (SDP: a=sendrecv)--|
|<-------------------------------200 OK-------------------------------|
|-------------ACK---------------------------------------------------->|
| | | | |
|=========== Normal RTP media resumes =================================|SIP / SDP Example for VoNR Hold and Resume
A simplified example is:
Hold Request
INVITE sip:userB@ims.example.com SIP/2.0
Content-Type: application/sdp
m=audio 49170 RTP/AVP 96
a=rtpmap:96 EVS/16000
a=sendonlyHold Response
SIP/2.0 200 OK
Content-Type: application/sdp
m=audio 50020 RTP/AVP 96
a=rtpmap:96 EVS/16000
a=recvonlyResume Request
INVITE sip:userB@ims.example.com SIP/2.0
Content-Type: application/sdp
m=audio 49170 RTP/AVP 96
a=rtpmap:96 EVS/16000
a=sendrecvThis reflects the standardized SIP offer and answer model: hold through sendonly and acknowledgment with recvonly, then resume by restoring a normal media direction such as sendrecv.
In some cases, inactive appears when neither side should send media.
What Changes in the 5G Network During Hold?
From a VoNR perspective, the important change is usually session and media state, not a complete re-establishment of IMS service.
So, in operational terms, engineers expect:
- the IMS dialog to remain established
- the SDP media direction to change
- the voice bearer and QoS context to remain service-aware, although packet flow behavior may change depending on endpoint or network implementation
- optional MRF/tone insertion if the operator supports music-on-hold or announcements through IMS resources
Important Parameters to Check
- SIP method and dialog continuity: Check that the hold and resume happens in the existing dialog, usually with re-INVITE and a clean 200 OK / ACK exchange.
- SDP direction attributes: Look for
a=sendonly,a=recvonly,a=inactive, anda=sendrecv. - Routing path through IMS: Verify traversal across P-CSCF and S-CSCF, and where applicable TAS.
- Media behavior after signaling: After hold, RTP should pause or become one-way according to negotiated SDP. After resume, bidirectional media should return.
- Supplementary-service handling: If operator features such as tones or announcements are involved, inspect TAS and MRF behavior too.
Common VoNR Hold / Resume Failure Scenarios
1. Hold Request Sent, but Far End Still Hears Audio
This often points to SDP direction handling problems, delayed media-path enforcement, or endpoint implementation issues rather than basic registration failure.
2. re-INVITE Fails with SIP Error
Check IMS routing, supplementary-service policy, and whether TAS or interworking functions are rejecting the update.
3. Hold Works, but Resume Fails
Look closely at the second SDP offer and answer exchange. In many implementations, the endpoint which placed the stream on hold is the one expected to resume it.
4. PSTN Interworking Behaves Differently
When the far side is on PSTN, hold and resume can involve BGCF, MGCF, and media-gateway interactions rather than a pure UE-to-UE SIP flow.
Summary
VoNR Call Hold / Resume is best understood as an IMS session modification inside an already established 5G voice call.
The NR access stays part of the active service context, while SIP and SDP changes the media direction.
In most traces, the heart of the procedure is simple: re-INVITE → SDP hold/resume attributes → 200 OK → ACK → media state change.
Standards References
- 3GPP TS 24.610 – Communication HOLD using IMS
- 3GPP TS 24.628 – Common Basic Communication procedures using IMS
- RFC 6337 – SIP Usage of the Offer/Answer Model
- 3GPP TS 23.228 – IMS architecture and procedures