What are SIP Methods / Requests and Responses?
SIP uses Methods / Requests and corresponding Responses to establish a call session.
SIP Requests:
There are six basic request / method types:
INVITE = Establishes a session
ACK = Confirms an INVITE request
BYE = Ends a session
CANCEL = Cancels establishing of a session
REGISTER = Communicates user location (host name, IP)
OPTIONS = Communicates information about the capabilities of the calling and receiving SIP phones
SIP responses:
SIP Requests are answered with SIP responses, of which there are 6 classes:
1xx = informational responses, such as 180, which means ringing
2xx = success responses
3xx = redirection responses
4xx = request failures
5xx = server errors
6xx = global failures
Note the similarity with HTTP – the beauty of SIP is in its clarity and simplicity
IP PBX, SIP & VOIP common questions 
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