SIP 100 Trying vs 406 Not Acceptable
SIP 100 (Trying) is a 1xx Provisional response, while 406 (Not Acceptable) 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, 406 means that the resource identified by the request is only capable of generating responses with content characteristics not acceptable according to the Accept header.
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 resource identified by the request is only capable of generating responses with content characteristics not acceptable according to the Accept header.
When You See It
When content negotiation fails — the server cannot produce a response matching the Accept header fields in the request.
How to Fix
Review the Accept headers in your request and broaden the acceptable content types.
Key Differences
100 is a 1xx Provisional response, while 406 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 406: The resource identified by the request is only capable of generating responses with content characteristics not acceptable according to the Accept header.
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 406 when when content negotiation fails — the server cannot produce a response matching the Accept header fields in the request.
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 406 (Not Acceptable): Review the Accept headers in your request and broaden the acceptable content types.