Difference Between 180 Ringing and 183 Session

In general, ringing is controlled via two Informational Responses in SIP: the 180 Ringing and the 183 Session Progress.

The main difference between them, is the 180 Ringing message instructs the UA to create the dial-tone locally, whereas the 183 Session Progress contains an SDP, which allows for regional ring-back and carrier announcements as well.

Typically 183 contains SDP and is used to play media before the call is connected