What's the cheapest carrier for Iphone?

Well, my parents are thinking in buying the Iphone 4 for the 3 of us. But they want unlimited texting and calling. We right now have Metro PCS, and it sucks. Right now, they pay 2 androids and 1 basic phone for $130.So, what carrier should we move to for unlimited texting and calling Iphone?

At&T because they have the best plans for cheap. They gave us 400$ because we were a long time user. Their phones are also pretty dang cheap so I would say go to At&T. You can even sell your phones their to get great phones from them! Their Better than most carriers.