Basically, none.
However VoIP is a very general word, to cover a whole host of technologies. Some are open standard and some oare closed.
Some VoIP systems convert your calls to PSTN (normal type calls) and others are purely between compatible devices..
It's a very big, but quite interesting field. For exapmple, I can call my mobile from a pc client or from a traditional line that dialls a landline number. Far to complex to expalin here!
Only the truly stupid believe they know everything.
Stu.. 2004