Apple Iphone 4s 16gb At&T Without Contract Phone?

I'm 16 years old and my Current phone just Broke which Was a little Payphone and My sister got a Unlocked Iphone 4s and made that into a payphone, I want to do the same but I was wondering can I do that with a No contract phone? Or do I just Flat out need a Unlocked one?

No you do not need to buy an unlocked phone unless it's main carrier isn't at&t(I believe you are speaking of the GO phones). If not you can make any iPhone work on any other GSM pre-paid if it is unlocked.

Right now, only Virgin Mobile offers no contract, iPhones. But if you want the unlocked iPhone from another company, you need to pay like $600.