SIP 100 Trying vs 428 Use Identity Header
SIP 100 (Trying) is a 1xx Provisional response, while 428 (Use Identity Header) 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, 428 means that the server requires an Identity header to verify the caller's identity but it was not included in the request.
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 server requires an Identity header to verify the caller's identity but it was not included in the request.
When You See It
In environments that enforce SIP Identity (RFC 4474) for caller authentication and spam prevention (STIR/SHAKEN).
How to Fix
Add a valid Identity header signed by an authentication service. Ensure your STIR/SHAKEN infrastructure is properly configured.
Key Differences
100 is a 1xx Provisional response, while 428 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 428: The server requires an Identity header to verify the caller's identity but it was not included in the request.
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 428 when in environments that enforce SIP Identity (RFC 4474) for caller authentication and spam prevention (STIR/SHAKEN).
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 428 (Use Identity Header): Add a valid Identity header signed by an authentication service. Ensure your STIR/SHAKEN infrastructure is properly configured.