VoNR Call Flow and Procedures | Voice Over NR Complete Tutorial

Introduction to VoNR

Voice over New Radio (VoNR) is the native voice service for 5G Standalone (SA) networks.

VoNR enables voice calls to be carried directly over 5G NR radio and the 5G Core network, without relying on legacy LTE fallback mechanisms.

Unlike VoLTE, where voice services operate over LTE radio access, VoNR allows pure 5G voice communication, delivering:

• lower call setup latency
• improved voice quality
• better spectrum efficiency
• simplified network architecture

VoNR uses the IP Multimedia Subsystem (IMS) for session control and SIP signaling.

The VoNR architecture is standardized by the 3rd Generation Partnership Project in specifications such as:

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

VoNR Architecture

VoNR integrates 5G NR radio access with the IMS core network to provide voice services.

How VoNR Works

1. UE connects to the 5G NR network
2. UE establishes an IMS PDU session
3. UE registers with IMS using SIP
4. Voice calls are handled using SIP signaling

The voice media is carried using RTP over the 5G user plane.

VoNR Deployment

Operators deploy VoNR when they roll out 5G Standalone networks.

There are typically three deployment stages:

VoNR Deployment Requirements

For VoNR to work, the following components must support VoNR:

• 5G Standalone Core Network
• IMS platform
• VoNR-capable smartphones
• NR coverage for voice services

Why Operators Deploy VoNR

Benefits include:

• lower voice latency
• simplified voice architecture
• improved spectrum utilization
• better integration with 5G services

Many operators initially deploy EPS fallback before migrating fully to VoNR.

VoNR Call Flow Procedures

This tutorial series explains all VoNR signaling procedures used in 5G networks.

Each procedure is explained in a dedicated tutorial with detailed call flows and diagrams.

1. VoNR Registration

IMS registration over the 5G NR network.

Topics covered:

• IMS PDU session establishment
• SIP REGISTER procedure
• VoNR readiness

Read tutorial: VoNR Registration Procedure

2. VoNR Mobile Originated Call

Procedure when the UE initiates a voice call.

Topics covered:

• SIP INVITE flow
• call routing via IMS
• media session establishment

Read tutorial: VoNR Mobile Originated Call Flow

3. VoNR Mobile Terminated Call

Procedure when the UE receives an incoming call.

Topics covered:

• network paging
• SIP INVITE delivery
• call acceptance

Read tutorial: VoNR Mobile Terminated Call Flow

4. VoNR Call Termination

Procedure used to end a voice call session.

Topics covered:

• SIP BYE signaling
• session release
• media termination

Read tutorial: VoNR Call Termination Procedure

5. VoNR Call Hold / Resume

Procedure used when a user temporarily pauses or resumes a call.

Topics covered:

• SIP re-INVITE signaling
• SDP modification
• session updates

Read tutorial: VoNR Call Hold and Resume

6. VoNR Emergency Call

Special procedure used for emergency voice services.

Topics covered:

• emergency SIP INVITE
• priority network routing
• emergency service handling

Read tutorial: VoNR Emergency Call Flow

7. EPS Fallback

Fallback mechanism used when VoNR is unavailable.

Topics covered:

• handover to LTE
• voice call over VoLTE
• fallback triggers

Read tutorial: EPS Fallback Procedure

VoNR vs VoLTE

VoNR represents the future evolution of voice services in mobile networks.

Summary

VoNR enables native voice services in 5G Standalone networks, using IMS signaling and NR radio access.

Key VoNR procedures include:

Together, these procedures form the complete VoNR call flow framework used in 5G networks.