SIP 100 Trying vs 415 Unsupported Media Type
SIP 100 (Trying) is a 1xx Provisional response, while 415 (Unsupported Media Type) 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, 415 means that the server is refusing the request because the message body is in a format not supported by the server for the requested method.
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 is refusing the request because the message body is in a format not supported by the server for the requested method.
When You See It
When the Content-Type of the request body is not supported. For INVITE, this often means an unsupported SDP format.
How to Fix
Check the Accept header in the response to see supported media types. Use a compatible Content-Type in your request.
Key Differences
100 is a 1xx Provisional response, while 415 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 415: The server is refusing the request because the message body is in a format not supported by the server for the requested method.
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 415 when when the Content-Type of the request body is not supported. For INVITE, this often means an unsupported SDP format.
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 415 (Unsupported Media Type): Check the Accept header in the response to see supported media types. Use a compatible Content-Type in your request.