Hello. I plan on purchasing an iphone to use for tmobile. Ive had att service and personally i think tmobile is 100x better. Now, i realize that if something were to go wrong with my iphone, i would not be able to get it serviced under warranty. But if i were to get a friend to take it in (she has att and an iphone) and put her sim card in and say it is hers, would they take it back? or does the Sim card have to match the serial number of the iphone?