okay you need 5 white candles, 2 red candles, 2 liters of cows blood (fresh) and a live chicken… do you know how to draw a pentagram?
just kidding. it really depends on what you want to do.
If you want to make int’l calls to normal phones, you need a voip carrier. broadvoice and viatalk and quantumvoice are good choices. this will cost you about $20/mo. You can also get a prepay carrier like voicepulse connect and they will only charge you per-minute.
If you can install some stuff on the other end (IE if this is for relatives or family) it becomes easier and cheaper. start by buying them an IP phone- grandstream bt200 is a good choice.
At that point you can either register the phone to your server directly, in which case you can dial each other by extension and not much else; or you can get two accounts on a free VoIP network like FWD (free world dialup) or SIPphone. If you do that, register both * and their phone to that network, and you can then call each other.
hope that helps!