SIP 100 Trying vs 401 Unauthorized
SIP 100 (Trying) is a 1xx Provisional response, while 401 (Unauthorized) is a 4xx Client Failure response. 100 indicates that the request has been received by the next-hop server and an unspecified action is being taken to process it. The UAC should expect additional responses. In contrast, 401 means that the request requires user authentication. The response includes a WWW-Authenticate header with a challenge for the realm.
Description
The request has been received by the next-hop server and an unspecified action is being taken to process it. The UAC should expect additional responses.
When You See It
Immediately after sending an INVITE. The proxy has received the call and is searching for the callee. This is the SIP equivalent of a dial tone.
How to Fix
No fix needed — this is normal call setup behavior. If stuck on 100 Trying for too long, check network connectivity and proxy configuration.
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
100 is a 1xx Provisional response, while 401 is a 4xx Client Failure response.
SIP 100: The request has been received by the next-hop server and an unspecified action is being taken to process it. The UAC should expect additional responses.
SIP 401: The request requires user authentication. The response includes a WWW-Authenticate header with a challenge for the realm.
You encounter 100 when immediately after sending an INVITE. The proxy has received the call and is searching for the callee. This is the SIP equivalent of a dial tone.
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 100 (Trying): No fix needed — this is normal call setup behavior. If stuck on 100 Trying for too long, check network connectivity and proxy configuration. For 401 (Unauthorized): Resend the request with proper Authorization header containing valid credentials (Digest authentication).