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Can you buy an iPhone and just put the sim card from your existing ATT 3G phone in I did so in my brother's iPhone and got service and wanted to know if there was a way to do this with a new iphone?

An AT&T 3G sim card will work in a new iPhone. yessss you can :) but the box for the iPhone comes with a little tool to help you open the little compartment for the sim (MORE)