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VoNR Mobile Terminated Call Procedure

call-flow VoNR | MT Call | Paging | IMS | RTP | 5G SA

VoNR Mobile Terminated Call is the incoming-call procedure where the network has to find the called UE, restore service if needed, and then deliver the IMS voice session.

That means MT analysis is never only about SIP. It combines paging, reachability recovery, and IMS dialog setup into one end-to-end user journey.

Introduction

When a voice call is terminated toward a VoNR-capable UE, the network first checks whether the subscriber is already active enough to receive SIP signaling.

If not, the AMF and gNB page the UE and restore service context before the incoming INVITE can be processed. Only then does the normal alerting and answer sequence begin.

What Is VoNR Mobile Terminated Call in Simple Terms?

What starts the procedure: An incoming IMS voice call is routed toward the called UE.
What the UE and network want to achieve: Restore reachability if needed, deliver the call, and establish media cleanly.
What success looks like: The phone rings, the user answers, and the voice session becomes active.
What failure means: Paging, service restoration, SIP delivery, or media continuity breaks before the call becomes usable.

Why this procedure matters

MT call delivery is one of the clearest proofs that registration, paging, and IMS service continuity really work together in the live network.

Quick Fact Sheet

Procedure name	VoNR Mobile Terminated Call
Domain	Native 5G incoming voice call delivery over IMS and NR
Main trigger	The called UE receives an incoming voice session while already provisioned for VoNR
Start state	The called UE is reachable in 5GS and registered in IMS, but may be in RRC Idle or RRC Inactive
End state	The UE is paged if needed, the SIP dialog completes, and voice media starts
Main nodes	Called UE, AMF, gNB, IMS core, remote calling side
Main protocols	Paging, service restoration, SIP INVITE, SDP, RTP
Main success outcome	Paging succeeds, INVITE reaches the UE, 200 OK and ACK complete, and media flows
Main failure outcome	The call is lost because paging, service restoration, IMS routing, or media negotiation fails
Most important messages	Paging, Service Request or Resume, INVITE, 180 Ringing, 200 OK, ACK
Main specs	TS 23.502, TS 24.501, TS 24.229, TS 38.331

Preconditions

The called UE is registered in 5GS and has a valid IMS registration.
The network has enough subscriber reachability context to locate the UE.
Paging and service-restoration handling are available when the UE is not already connected.
The voice media path and QoS support can be established after answer.

Nodes and Interfaces

Nodes involved

Node	Role in this procedure
Calling side and IMS core	Generate the incoming voice session and route it through the IMS signaling path toward the called UE.
AMF	Locates the called UE and starts paging or service-restoration handling if the UE is not already active.
gNB	Broadcasts paging on NR and restores radio connectivity before SIP call delivery.
Called UE	Responds to paging, restores service state, receives INVITE, and accepts or rejects the call.

Interfaces used

Interface	Path	Role
NR-Uu	UE <-> gNB	Carries paging reception, service restoration, and later voice radio continuity.
N1 and N2	UE/gNB <-> AMF	Carry service request or resume handling after paging.
IMS SIP path	IMS core <-> called UE	Carries INVITE, progress responses, answer, and ACK.
User-plane RTP path	Calling side <-> called side	Carries voice media after the SIP dialog is established.

End-to-End Call Flow

Remote side        IMS / AMF / gNB        Called UE
|-- incoming call --->|                     |
|                     |-- Paging --------->|
|                     |<-- Service restore -|
|--------------------- INVITE ------------->|
|<-------------------- 180 Ringing ---------|
|<-------------------- 200 OK --------------|
|--------------------- ACK ---------------->|
|=============== RTP voice media ===========|

Major Phases

Phase	What happens
1. Incoming call reaches IMS	The remote side generates a new SIP dialog for the called subscriber.
2. Reachability recovery	If the UE is idle or inactive, paging and service restoration must happen first.
3. SIP call delivery	The INVITE reaches the UE and the called side starts alerting.
4. Answer and media establishment	200 OK, ACK, and RTP complete the incoming call setup.

Step-by-Step Breakdown

IMS identifies the called subscriber and checks reachability

Sender -> receiver: Remote IMS side -> AMF and called-side context

Message(s): Incoming INVITE and subscriber reachability handling

Purpose: Decide whether the called UE is already active or must first be paged and restored.

State or context change: The network turns a generic incoming call into a UE-specific reachability problem.

Note: Many MT issues happen before the INVITE reaches the UE because the called side is not yet restored for service.

AMF and gNB page the UE if it is idle or inactive

Sender -> receiver: AMF -> gNB -> UE

Message(s): Paging and follow-on service restoration

Purpose: Bring the UE back into a state where IMS signaling can be delivered reliably.

State or context change: The called UE transitions from background reachability into active service handling.

Note: Separate paging failure from SIP failure early. If paging breaks, the voice dialog never gets a fair chance.

The called UE receives INVITE and returns provisional progress

Sender -> receiver: IMS core -> called UE

Message(s): INVITE, 100 Trying, 180 Ringing

Purpose: Deliver the incoming call and confirm that the user is being alerted.

State or context change: The session is now visible to the called subscriber but not yet answered.

Note: Ringing proves much more than basic registration. It proves that paging, service restoration, and IMS delivery all worked together.

The called UE answers and voice media starts

Sender -> receiver: Called UE -> remote side

Message(s): 200 OK, ACK, RTP

Purpose: Complete the incoming dialog and begin the actual voice session.

State or context change: The MT call leaves alerting state and becomes an active bidirectional voice call.

Note: Final success means both signaling and media continuity are clean. A 200 OK without good media is only partial success.

Important Messages in This Flow

Message	Protocol	Direction	Purpose in this procedure	What to inspect briefly
Paging	RRC	AMF / gNB -> UE	Recover reachability for the called UE.	Check the paging identity and whether the UE responded in time.
Service Request	NAS	UE -> AMF	Restore active service continuity after paging.	Useful when the UE was idle and needed uplink restoration before SIP delivery.
INVITE	SIP	IMS -> called UE	Deliver the incoming call setup request.	Inspect Request-URI, SDP, and whether it arrived only after service restoration was complete.
180 Ringing	SIP	Called UE -> IMS	Shows the user is being alerted.	If missing, inspect UE alerting logic and remote routing.
200 OK	SIP	Called UE -> remote side	Confirms call acceptance.	Inspect the negotiated media profile and codec answer.

Important Parameters to Inspect

Parameter	What it is	Where it appears	Why it matters	Common issues
Reachability state	Whether the UE is connected, idle, or inactive before the call.	Before paging and INVITE delivery	Explains whether the MT case is mainly a reachability problem or directly an IMS problem.	Misreading the initial state sends troubleshooting to the wrong layer.
Paging identity	Identifier used to page the called subscriber.	Paging	Confirms the network is really targeting the expected UE.	Identity mismatch leads to a silent paging failure.
Service restoration timing	How long it takes the UE to become active enough for SIP delivery.	Paging and NAS restore stage	Critical for incoming-call setup delay and missed-call behavior.	Slow restoration can make IMS timers expire.
INVITE SDP offer	The media proposal for the incoming session.	INVITE	Needed to understand codec compatibility and early media handling.	Malformed or incompatible SDP breaks the session after successful reachability.
Answer and ACK continuity	Whether the final call-accept path really completes.	200 OK and ACK	Confirms the session moved from alerting into a stable established call.	Missing ACK creates half-open answered-call symptoms.

Success Criteria

The called UE is restored into a service-ready state in time.
The INVITE reaches the called UE and alerting starts normally.
The answer path completes with 200 OK and ACK.
Media continuity is stable after the call is answered.

Common Failures and Troubleshooting

Symptom	Likely cause	Where to inspect	Relevant message(s)	Relevant interface(s)	Likely next step
Paging fails and the phone never rings	The called UE was not successfully restored for service.	Paging identity, UE response, and service-request timing.	Paging, Service Request	NR reachability	Do not misclassify this as an IMS routing problem if the UE never became active.
Ringing appears but answer does not complete	The call reached the UE, but final SIP answer handling failed.	INVITE, 180 Ringing, 200 OK, and ACK continuity.	180 Ringing, 200 OK	IMS SIP path	This is later in the flow than pure paging trouble.
Call answers but audio is poor or absent	Media negotiation or QoS continuity failed after signaling success.	SDP answer, RTP path, and QoS treatment.	200 OK, RTP start	User plane	Treat this as post-setup media failure, not call-delivery failure.
Incoming-call setup is slow	Reachability recovery or IMS routing is taking too long.	Paging-to-service timing and provisional SIP progress.	Paging, INVITE, 180 Ringing	Cross-layer timing	Timing analysis is usually more useful than single-message inspection here.

What to Check in Logs and Traces

Start by classifying the called UE as connected, idle, or inactive.
Separate paging success from later SIP call delivery.
Use Ringing versus no Ringing as a strong split between reachability and later IMS issues.
After answer, switch quickly to media and QoS inspection if user symptoms are audio-related.

Related sub-procedures

Related message reference pages

Related troubleshooting pages

FAQ

What is a VoNR Mobile Terminated Call?

It is the incoming-call flow where the network must reach the called UE, restore service if needed, and then complete IMS call delivery over 5G.

Why is paging important here?

Because many called UEs are not already in a fully active radio state when the incoming call arrives.

What proves success?

The UE is paged successfully, the call rings, 200 OK and ACK complete, and RTP media starts normally.

What should I inspect first?

Start with the called UE reachability state, then paging and service restoration, and only then move into SIP call-delivery analysis.

Why can the call ring but still fail?

Because final answer handling, media negotiation, or post-answer voice continuity can still break after the alerting stage.