SIP is a simple, ASCII-based protocol that uses requests and responses to establish communication among the various components in the network and to ultimately establish a conference between two or more endpoints.
There are a few call flow scenarios in a SIP based network:
• SIP Gateway-to-SIP Gateway—Call Setup and Disconnect
• SIP Gateway-to-SIP Gateway—Call via SIP Redirect Server
• SIP Gateway-to-SIP Gateway—Call via SIP Proxy Server
In the case of the gateway-to-gateway call setup and disconnect via a proxy server, the following steps are followed:

The caller user agent sends an INVITE request to the proxy server, the proxy server determines the path, and then forwards the request to the called party.

The called party responds to the proxy server, which in turn, forwards the response to the caller.

The proxy server forwards the acknowledgments of both parties. A session is then established between the caller and called party. Real-time Transfer Protocol (RTP) is used for the communication between the caller and the called party.

Example SIP Call Flow

Related Terms:SIP call flow, SIP call setup

Reference Links:http://www.javvin.com/protocolVOIP.html : VOIP Protocols