IMS 5G AND LTE IMS network -> UE RFC 3261 / 3GPP TS 24.229

IMS SIP 380 Alternative Service

380 Alternative Service indicates that the original request cannot be served as asked and suggests another service path.

Message Fact Sheet

Protocol	ims	Network	5G and LTE
Spec	RFC 3261 / 3GPP TS 24.229	Spec Section	SIP 380 Alternative Service redirection response
Direction	IMS network -> UE	Message Type	SIP redirection response

Full message name	IMS SIP 380 Alternative Service
Protocol	IMS
Technology	5G and LTE
Common deployment	VoNR and VoLTE
Direction	IMS network -> UE
Interface	Gm with onward IMS routing
Signaling bearer / channel	IMS SIP signaling / SIP service-redirection response
Typical trigger	The requested service is not available on the current branch, but an alternative service should be used.
Main purpose	Points the sender toward an alternate service choice instead of accepting the current request branch.
Main specification	RFC 3261 / 3GPP TS 24.229, SIP 380 Alternative Service redirection response
Release added	See specification history
Procedures where used	IMS Service Redirection

What is IMS SIP 380 Alternative Service in simple terms?

380 Alternative Service indicates that the original request cannot be served as asked and suggests another service path.

Points the sender toward an alternate service choice instead of accepting the current request branch.

Why this message matters

380 means the original service path is not being used and an alternative service should be considered.

Where this message appears in the call flow

IMS Service Redirection

Call flow position

Previous message(s): IMS SIP INVITE

Next message(s): Retried request or alternate-service handling

Message direction and transport

Sender and receiver: IMS network -> UE

Interface: Gm with onward IMS routing

Domain: IMS service routing

Signaling bearer: IMS SIP signaling

Logical channel: SIP service-redirection response

Transport / encapsulation: SIP over UDP, TCP, or TLS

Security context: Uses the same protected IMS signaling path as the original request.

Message Structure Overview

380 is about alternative service selection rather than direct request acceptance.

ASN.1 Message Syntax for IMS SIP 380 Alternative Service

SIP/2.0 380 Alternative Service
Via:
From:
To:
Call-ID:
CSeq:
Contact:                           OPTIONAL
Content-Length: 0

How to read this message syntax

This is SIP response syntax rather than ASN.1.

IMS SIP 380 Alternative Service - Example Dump

SIP/2.0 380 Alternative Service
Via: SIP/2.0/UDP ue.example.net;branch=z9hG4bK-inv1
From: <sip:alice@example.net>;tag=mo1
To: <sip:bob@example.net>;tag=term1
Call-ID: call-001@example.net
CSeq: 1 INVITE
Content-Length: 0

Important Information Elements

IE	Required	Description
`Status-Line`	Yes	Shows the 380 redirection code.
`Contact`	Optional	Can point toward alternate service handling.

Detailed field explanation

`Status-Line`

Shows the 380 redirection code.

Presence: Required

In practice: In practice, compare this field with the original request and with any later release-dependent optional fields so you can see whether the network accepted the same service model the UE asked for.

`Contact`

Can point toward alternate service handling.

Presence: Optional

Common Issues and Troubleshooting

The requested service is not accepted, but the transaction does not look like a simple failure.

Likely cause: The network is pointing the request toward alternative service logic.

What to inspect: Check whether alternate service details or later rerouting are present.

Next step: Follow the alternate service path rather than treating it as a final routing dead end.

FAQ

What does 380 Alternative Service mean?

It means the original service request is not accepted as-is and an alternative service path should be used.

Related Procedures

Related Tools

Use This Reference in Practice

Decode this message with the 3GPP Decoder, inspect the related message database, or open the matching call flow to see where this signaling step fits in the full procedure.