Buying an iPhone 4s on ebay? (AT&T)?

I currently have an android phone, and I want to buy an iPhone 4S. If I buy an AT&T iPhone on ebay, what do I have to do? I heard that iPhones need different sim cards, so do I have to buy a new one, and will it have the same number? Also, would I have to change my plan?

Once it comes in take it to your local AT&T store, theyll just put that phone on your plan and i believe give you a new sim card, i'm not sure if you have to pay for it though. And i'm 99% sure they can make it so you have the same number!

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