I've been thinking about this one since my post yesterday and realised you are'nt really talking about Voip as telephone techs would. When you say Voip (V oice Over Internet Protocol), what you really mean is voice over THE internet.
This is a completely different matter and is highly unlikely to become available for a long time in a truly useable way. The reason for this is due to the way information is propagated over IP. In order to have acceptable voice communications over IP, voice has to be given priority over anything else and it is obviously not in the interests of the service providers to provide this higher priority because for one thing it would chew up the bandwidth for the value added services the internet offers. Another reason is it would decrease revenue in the voice field where a lot of the big ISPs make thier money.
Hope this helps.
R4