I'm on an unlimited family data plan that has been grandfathered since the iPhone 3G. I'm thinking about getting an unlocked phone, but I want to keep my AT&T plan. Is this possible? I'd imagine that I would just need to buy an AT&T sim, but would this violate the plan? How much do the SIMs cost? Apologies if these are elementary questions - I'm used to my 2 year AT&T plans!