SIP 183 Session Progress vs 415 Unsupported Media Type
SIP 183 (Session Progress) is a 1xx Provisional response, while 415 (Unsupported Media Type) is a 4xx Client Failure response. 183 indicates that used to convey information about the progress of the call that is not otherwise classified. Often carries early media (ringback tones, announcements) via SDP. 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
Used to convey information about the progress of the call that is not otherwise classified. Often carries early media (ringback tones, announcements) via SDP.
When You See It
During call setup, especially in PSTN interworking scenarios. The response may contain SDP for early media like custom ringback tones or IVR prompts.
How to Fix
No fix needed — this is informational. If early media is not playing, check codec negotiation and SDP handling in your UA.
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
183 is a 1xx Provisional response, while 415 is a 4xx Client Failure response.
SIP 183: Used to convey information about the progress of the call that is not otherwise classified. Often carries early media (ringback tones, announcements) via SDP.
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 183 when during call setup, especially in PSTN interworking scenarios. The response may contain SDP for early media like custom ringback tones or IVR prompts.
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 183 (Session Progress): No fix needed — this is informational. If early media is not playing, check codec negotiation and SDP handling in your UA. 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.