Build you own SIP server : Asterisk

With these applications, you can configure your own IP PBX easily at office, or even at your own house! It is easier than you think~

Trixbox

It has FREEWARE! Now you no need to know and crack your head to figure out how to look into MANY MANY config files in linux...it is all GUI, for standard feature if you are looking for.

http://www.trixbox.org

AsteriskNOW

This is another open source freeware that just came across, an open source that built up with customized linux distributions that includes Asterisk! AsteriskNOW is much more easier than Trixbox, you can create your own voice sip server within a day! Easy to install, and offer flexibility, functionality...but it is not advanced type. If you are looking for some 'great' features, then it is not suitable for you.

http://www.asterisknow.org

If you are looking for Analog FXO/FXS Gateway ( What is FXO & FXS ? ), maybe you can try Digium or Planet for home user. Definately, you may go for Cisco as well...