SIP
183 Session Progress
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 This Code
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.