VoiceTime?

I have always wondered why this hasn't been done yet. We have iMessage, FaceTime, so why the hell not complete the package and offer a VoIP service on our iPhones? Its so very obvious, and should be very easy to implement. Although the FaceTime fiasco shows that getting this over 3G would be near impossible, it would be amazing to get. It would get us one step forward to making carriers dumb pipes, we could just choose data only plans, or plans with very few minutes(such as tmobiles 30 dollar plan) if most of our contacts had iPhones, which in my case, I do, and save so much money.