SIP 181 Call Is Being Forwarded vs 401 Unauthorized
SIP 181 (Call Is Being Forwarded) is a 1xx Provisional response, while 401 (Unauthorized) is a 4xx Client Failure response. 181 indicates that a proxy server may send this to indicate that the call is being forwarded to a different set of destinations. In contrast, 401 means that the request requires user authentication. The response includes a WWW-Authenticate header with a challenge for the realm.
Description
A proxy server may send this to indicate that the call is being forwarded to a different set of destinations.
When You See It
When the callee has call forwarding enabled and the request is being redirected to another number or device.
How to Fix
No action needed. The call is being redirected. If forwarding is unintended, the callee should update their forwarding settings.
Description
The request requires user authentication. The response includes a WWW-Authenticate header with a challenge for the realm.
When You See It
When making a request that requires Digest authentication. Common for REGISTER and INVITE requests to authenticated SIP trunks.
How to Fix
Resend the request with proper Authorization header containing valid credentials (Digest authentication).
Key Differences
181 is a 1xx Provisional response, while 401 is a 4xx Client Failure response.
SIP 181: A proxy server may send this to indicate that the call is being forwarded to a different set of destinations.
SIP 401: The request requires user authentication. The response includes a WWW-Authenticate header with a challenge for the realm.
You encounter 181 when when the callee has call forwarding enabled and the request is being redirected to another number or device.
You encounter 401 when when making a request that requires Digest authentication. Common for REGISTER and INVITE requests to authenticated SIP trunks.
When to Use Which
For 181 (Call Is Being Forwarded): No action needed. The call is being redirected. If forwarding is unintended, the callee should update their forwarding settings. For 401 (Unauthorized): Resend the request with proper Authorization header containing valid credentials (Digest authentication).