Simple. Don't even need to pay $3000/mo. Take that P3 you have sitting around, and put Asterisk@Home on it. http://asteriskathome.sourceforge.net/
Its a linux distribution that has Asterisk (the most popular open source PBX out there at the moment) and the Asterisk Management Portal to easily manage it from a control panel. Have a look for yourself.
Then goto a company like Nufone, Voipjet, Stanaphone, etc. and get cheap termination for as low as 1.2c/min.
Cost: Free box, Free PBX software, cheaper rates per minute