| Protocol | ims | Network | 5G and LTE |
|---|---|---|---|
| Spec | RFC 3261 / 3GPP TS 24.229 | Spec Section | SIP 181 Call Is Being Forwarded provisional response |
| Direction | IMS network -> UE | Message Type | SIP provisional response |
| Full message name | IMS SIP 181 Call Is Being Forwarded |
|---|---|
| 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 / Provisional session-routing response |
| Typical trigger | The called side or network applies call forwarding before final answer. |
| Main purpose | Explains why the dialog is changing route or destination before a final response arrives. |
| Main specification | RFC 3261 / 3GPP TS 24.229, SIP 181 Call Is Being Forwarded provisional response |
| Release added | See specification history |
| Procedures where used | IMS Session Routing, VoNR Call Forwarding, VoLTE Call Forwarding |
181 Call Is Being Forwarded shows that the original target is redirecting or forwarding the session attempt.
Explains why the dialog is changing route or destination before a final response arrives.
181 means the original target is forwarding the call or session attempt.
Previous message(s): IMS SIP INVITE
Next message(s): 180 Ringing, 200 OK, 302 Moved Temporarily
Security context: Uses the same protected IMS signaling path as the INVITE transaction.
SIP/2.0 181 Call Is Being Forwarded
Via:
From:
To:
Call-ID:
CSeq:
Content-Length: 0
This is SIP response syntax rather than ASN.1.
SIP/2.0 181 Call Is Being Forwarded
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
| IE | Required | Description |
|---|---|---|
Status-Line | Yes | Shows the 181 response code and meaning. |
Call-ID | Yes | Correlates the forwarding indication with the dialog attempt. |
Status-LineShows the 181 response code and meaning.
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.
Call-IDCorrelates the forwarding indication with the dialog attempt.
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.
Likely cause: Forwarding changed the route before final answer.
What to inspect: Check whether 181 appeared before later alerting or answer responses.
Next step: Follow the route set and next dialog branch after the forwarding indication.
It means the session attempt is being redirected or forwarded before final answer.
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.