In this section a call will be analyzed in detail. In a SIP call there are several SIP transactions. A SIP transaction consists of several requests and answers and the way to group them in the same transaction is by means of CSeq parameter.

- The first step is the user register. The users must register themselves to be found by other users. In this case, the terminals send a REGISTER request, where the fields "from" and "to" correspond to the registered user. The Proxy server, who acts as Register, consults if the user can be authenticated and sends an OK message if there is no problem.

-The following transaction corresponds to a session establishment. This session consists of an INVITE request of the user to the proxy. Immediately, the proxy sends a TRYING 100 to stop the broadcastings and reroute the request to the B user. The B user sends a Ringing 180 when the telephone begins to ring and it is also reroute by the proxy to the A user. Finally, the OK 200 message corresponds to the accept process (the user B response the call).

-At this moment the call is established, and the RTP transport protocol starts with the parameters (ports, addresses, codecs, etc.) of the SDP protocol.

-The last transaction corresponds to a session end . This is carried out with an only BYE request to the Proxy, and later reroute to the B user. This user replies with an OK 200 message to confirm that the final message has been received correctly.