Is the T-Mobile iPhone 6 unlocked and 100% compatible with AT&T 4G LTE bands? I remember T-Mobile used to use a different network than AT&T even though both are GSM (AWS?). My question is... if I buy the T-Mobile iPhone 6 through Apple for $649, is it unlocked AND 100% compatible with AT&T? It's for my father, the whole family is on AT&T but he travels so he needs it to be unlocked.