"Right, SIP is an application protocol that runs on top of TCP.
There are many different ways to transfer voice over TCP/IP, however SIP is probably the most used and understood in the industry with the maximum compatibility.
If you just want to make calls from your computer, many come with the phone dialer application (not sure if it is included any more in Vista/7), but you should be able to find one online. This uses a standard modem and had nothing to do wit SIP, it is purely POTS (Plane old telephone system)."
A SIP trunk is a direct connection between your organization and an Internet telephony service provider. SIP trunk enables you to extend VoIP telephony beyond your organization’s firewall without the need for an IP-PSTN gateway. This simpler configuration is easier and less expensive to design, operate, maintain, and upgrade. Here is a suggested [blog]: http://www.dovecommunications.com/modern-voip-features-explained-sip-trunking/ where you can read in more details about SIP trunking.