I currently have an iPhone 5 with at&t on contract. My gf has my old iPhone 4 and uses it unlocked on air voice wireless (at&t mvno). Since T-Mobile requires you to trade in your old phone and purchase a new T-Mobile phone, can i just say that the iPhone 4 is my phone that is on "contract with at&t"? I can trade that in, purchase an iPhone 5s from T-Mobile and give my iPhone 5 to my gf. Is that possible and is there a way T-Mobile can actually check that the phone i trade in with them is the phone that is currently on contract? I mean I could literally give them an old nokia from 10 years ago and say I'm on contract with that right? Thanks for all your help people of macrumors!