SIP uses methods / requests and corresponding responses to communicate and establish a call session.
There are fourteen SIP request methods of which the first six are the most basic request / method types:
SIP requests are answered with SIP responses, of which there are six classes:
1xx = Informational responses, such as 180 (ringing).
2xx = Success responses.
3xx = Redirection responses.
4XX = Request failures.
5xx = Server errors.
6xx = Global failures.