I just got an iPhone 6S Plus from an AT&T store, then realized it is locked by AT&T even thought it is a A1634 model. Apparently only the AT&T version from Apple Store are unlocked. i just confirmed that this morning by insert my coworker's TMobile card and it was not working. i need to use this phone in mainland china in a couple months, is paying the device in full and ask AT&T to unlock the only choice i have? can i go to Apple Store and ask to swap for an AT&T model there?