SIP, which stands for Session Initiation Protocol, is a set of commands and protocols in order to facilitate voice and/or video communication. As part of the protocol, a SIP proxy server makes requests, also called queries, to another server. The following is a list of some of the possible queries

INVITE: An invitation to a call

ACK: Short for “acknowledged.” The client has confirmed the INVITE request

PRACK: A provisional acknowledgment. Not related to VoIP provisioning, PRACK refers to a less reliable confirmation.

CANCEL: Cancel any pending requests

BYE: The call has been completed

REFER: Asks the client to issue a SIP query. This is most often a call transfer

REGISTER: Communicates user location

MESSAGE: Sending Business SMS over SIP

SUBSCRIBE: Subscribes to events

PUBLISH: Publishes an event to a server

NOTIFY: Notifies all subscribers of an event

INFO: Sends mid-session information that does not modify the session’s state

OPTIONS: Queries for information about the capabilities of the calling and receiving SIP phones

SIP Requests are then answered with SIP responses. There are 6 classes of SIP responses:
1xx = informational responses (Examples: 100 Trying, 180 Ringing, 181 Call is Being Forwarded)
2xx = success responses (Examples: 200 OK 202 Accepted)
3xx = redirection responses (Examples: 300 Multiple Choices, 301 Moved Permanently, 32 Moved Temporarily)
4xx = request failures (Examples: 403 Forbidden, 404 Not Found, 406 Not Accepted)
5xx = server errors (Examples: 502 Bad Gateway, 504 Server Time Out)
6xx = global failure (Examples: 600 Busy Everywhere, 604 Does Not Exist Anywhere)