In the peer to peer configuration shown in Figure 280, two SIP phones (in the example, FortiFones) communicate directly with each other. The phones send SIP request and response messages back and forth between each other to establish the SIP session.

Peer to peer configurations are not very common because they require the SIP phones to keep track of the names and addresses of all of the other SIP phones that they can communicate with. In most cases a SIP proxy or re-direct server maintains addresses of a large number of SIP phones and a SIP phone starts a call by contacting the SIP proxy server.