I would read the TOS of your ISP very closely, mine state that i cannot sell all or part of my service. This would indicate (if the case with your contract with your ISP) that you cannot make profit from selling VOIP on their network. If it is for personal use i dont know how they'd feel, if they are the provider of your phone servie they may cop the hump and try throttling bandwidth and/or refusing connections on certain ports.
Gizmo (and several others) can use the SIP protocol, there may be a way to translate sip to phone (is it PBX? i'm ouot of my depth here) I've heard asterix can do it, but i thought that was internal systems, not accross public phone networks.
edit - Skypeout - PC to phone, is free in N America right now i believe (for a month or 2 at least)
Last edited by ethics; 08-01-2006 at 10:45 AM.