sip:180 ringing vs 183 Session Progress

If You know this phone is ringing (an ALERT q.931 message, for instance) you send a ringing.

If You receive a notification indicating then the call was progressing, but don't know for sure whether the user I s being alerted or not, you send a 183 Session Progress message.

Both can indicate early media with SDP. If There is no SDP, the end device (softphone/gateway/etc.) have to generate the ringback tone or progress tone.

Usually you'll see the without SDP while 183 with SDP. It's a good practice to leave the tone generation for the endpoints.

If you get 183 should open media connection because there are audio ready for them to hear.

If you set ringback var and ignore_early_media, both and 183 would trigger your fake ringing. If you set instant_ringback=true then it would not wait for 18x it would start fake ringback instant (asterisk mode).

RFC3261:

21.1.2 ringing

The UA receiving the INVITE is trying to alert the user. This

Response may used to initiate local ringback.

21.1.5 183 Session Progress

The 183 (Session Progress) response is used to convey information

About the progress of the otherwise classified. The

Reason-phrase, header fields, or message body could used to convey

More details on the call progress.