I purchased an unlocked T-Mobile 6+ from Apple and have used it both on the Consumer Cellular and AT&T network (the latter using the sim from my iPhone 5). When I travel overseas my plan is to replace the sim with a T-Mobile one for the time that I am traveling. Since I am grandfathered in with an unlimited data plan I actually also purchased an AT&T 6+ (identical to the T-Mobile phone) from Apple which should arrive shortly. My plan is to sell this phone with the box unopened. I would like to sell it as an "unlocked" phone, but I am not sure when and where the locking occurs. If I were to fire up the new phone I assume that it would become locked. My questions are: 1) how does the locking occur? Since the AT&T phone is initially unlocked when purchased from Apple, does it happen when it is first used? Is the locking in the SIM, the phone, or a combination of both? 2) Once this phone arrives am I at risk of having my unlocked T-Mobile phone becoming locked while using using the old iPhone 5 sim? I.E., once I start the new contract, will AT&T somehow they lock my unlocked phone? Since I will be under contract, they will think that the phone is still "theirs".