VoNR Emergency Call Procedure | Voice Over NR Emergency Services Tutorial

Introduction

The VoNR Emergency Call procedure enables a user to place an emergency voice call over a 5G Standalone network.

Emergency calls are given highest priority in the network, ensuring that users can contact emergency services even during network congestion.

In 5G networks, emergency calls are handled through the IP Multimedia Subsystem (IMS) and routed to the Public Safety Answering Point (PSAP).

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

• 3GPP TS 23.167 – IP Multimedia Emergency Sessions
• 3GPP TS 23.502 – 5G Procedures
• 3GPP TS 24.229 – SIP signaling for IMS

Network Elements Involved

Several network elements participate in the VoNR emergency call flow.

VoNR Emergency Call Signaling Flow

UE        gNB        AMF        IMS/E-CSCF        PSAP
 |         |          |            |               |
 |---Emergency INVITE---------------------------->|
 |         |          |            |               |
 |<-------100 Trying------------------------------|
 |<-------180 Ringing-----------------------------|
 |<-------200 OK----------------------------------|
 |---------ACK----------------------------------->|
 |========== RTP Voice Media ====================|

Step-by-Step VoNR Emergency Call Procedure

Step 1: Emergency Call Trigger

The procedure begins when the user dials an emergency number such as:

The UE recognizes the dialed number as an emergency service request.

The device generates a SIP INVITE with emergency indicators.

Step 2: SIP Emergency INVITE

The UE sends a SIP INVITE message through the IMS PDU session.

The SIP INVITE contains:

• emergency service indication
• caller identity
• location information

Example header fields:

Request-URI: urn:service:sos
Priority: emergency

Step 3: Emergency Call Routing

The IMS core network routes the emergency call to the Emergency Call Session Control Function (E-CSCF).

The E-CSCF performs:

• emergency service validation
• routing to the nearest PSAP
• priority resource allocation

Step 4: Call Setup

The PSAP system processes the call and returns SIP responses.

Typical responses include:

Step 5: Voice Session Establishment

After the SIP handshake:

• RTP voice media begins
• the user communicates directly with the emergency operator

Voice packets are transmitted through the 5G user plane via the UPF.

Emergency Call Priority Handling

Emergency calls receive highest network priority.

Key mechanisms include:

QoS Configuration for Emergency Calls

Emergency voice calls use dedicated QoS profiles.

This ensures minimal latency and reliable voice communication.

Emergency Call Location Support

The UE may send location information to the network.

This allows emergency services to locate the caller.

Location methods include:

Troubleshooting Emergency Call Issues

Emergency Call Not Completed

Possible causes:

• IMS routing failure
• incorrect emergency number recognition
• network configuration issue

Poor Voice Quality

Possible causes:

• radio coverage issues
• packet loss
• congestion

Incorrect PSAP Routing

Possible reasons:

• incorrect location information
• misconfigured emergency routing rules

Summary

The VoNR Emergency Call procedure allows users to contact emergency services directly through the 5G network.

The process includes:

1. UE sending Emergency SIP INVITE
2. IMS routing the call via E-CSCF
3. call delivery to the PSAP
4. RTP media session establishment

This mechanism ensures reliable emergency communications in 5G VoNR networks.