Hi, I'm currently on at&t and i have an upgrade coming up soon. I'm thinking to get the iPhone 6 with the upgrade and subsequently switch to T-Mobile and have them pay for the ETF. I'm going to use a cheap flip phone to trade in to T-Mobile and buy an iPhone 6 from T-Mobile for my sister's B-day. My question is, will this work? and will at&t unlock my iPhone 6 immediately upon paying the early termination fee? thats the only thing that seems to be keeping away from doing this. Thanks!