VoNR Registration Procedure | Voice Over NR IMS Registration Tutorial

Introduction

VoNR Registration is the process through which a User Equipment (UE) registers with the IP Multimedia Subsystem (IMS) using the 5G NR radio access network.

Before a UE can initiate or receive voice calls using Voice over NR (VoNR), it must complete:

1. 5G network registration
2. IMS PDU session establishment
3. IMS SIP registration

After successful registration, the UE becomes VoNR ready, meaning it can perform native voice calls over 5G Standalone networks.

The procedure is defined by the 3rd Generation Partnership Project in the following specifications:

• 3GPP TS 23.501
• 3GPP TS 23.502
• 3GPP TS 24.229

Network Elements Involved

The VoNR registration procedure involves several network components.

VoNR Registration Call Flow

The registration sequence spans both 5G core signaling and IMS signaling.

VoNR Registration Call Flow Diagram

UE        gNB        AMF        SMF        IMS
 |         |          |          |          |
 |--Registration Request------->|          |
 |         |----------N2-------->|          |
 |         |                     |          |
 |------PDU Session Establishment----------|
 |                                         |
 |-----------SIP REGISTER----------------->|
 |<----------401 Unauthorized--------------|
 |-----------REGISTER (Auth)-------------->|
 |<----------200 OK------------------------|

Step-by-Step VoNR Registration Procedure

Step 1: 5G Initial Registration

The UE first registers with the 5G Core Network.

This procedure establishes the basic connectivity between the UE and the network.

Important messages include:

After this step, the UE becomes registered in the 5G network.

Step 2: IMS PDU Session Establishment

To access IMS services, the UE establishes a PDU session dedicated for IMS signaling.

Typical configuration:

The Session Management Function (SMF) creates the session, while the User Plane Function (UPF) handles traffic routing.

This step allows the UE to communicate with IMS servers.

Step 3: SIP REGISTER Procedure

Once the IMS PDU session is active, the UE registers with IMS using SIP signaling.

The SIP registration process follows these steps:

1. UE sends SIP REGISTER to the P-CSCF
2. IMS responds with 401 Unauthorized
3. UE sends REGISTER with authentication credentials
4. IMS validates credentials and responds 200 OK

After this step, the UE becomes IMS registered.

Example SIP REGISTER Message

REGISTER sip:ims.operator.com SIP/2.0
From: <sip:user@operator.com>
To: <sip:user@operator.com>
Contact: <sip:user@UE-IP>
Authorization: Digest credentials

This message allows IMS to authenticate the subscriber and create a registration binding.

VoNR QoS Configuration

VoNR services rely on QoS flows to ensure low latency voice traffic.

Typical QoS identifiers include:

These QoS flows guarantee low delay and high reliability for voice sessions.

Important Parameters to Check

For troubleshooting VoNR registration, engineers should verify:

Troubleshooting VoNR Registration Issues

IMS Registration Failure

Possible causes:

• incorrect IMS configuration
• unreachable P-CSCF
• subscriber profile mismatch

PDU Session Failure

Possible reasons:

• IMS DNN not allowed
• SMF configuration error
• incorrect UE subscription profile

SIP Authentication Failure

Possible causes:

• incorrect credentials
• authentication vector mismatch
• subscriber database error

Registration Timing

Typical VoNR registration times:

Total time for VoNR readiness is usually less than 1 second.

Summary

The VoNR Registration procedure prepares the UE for voice services in 5G networks.

The process includes:

1. 5G network registration
2. IMS PDU session establishment
3. SIP registration with IMS

Once these steps are complete, the UE is VoNR capable and ready to initiate or receive voice calls over 5G NR.