Hi there - I know has probably been asked before... I am selling my unlocked T-Mobile iPhone 6 Plus to a friend and they are on AT&T. I am sure it will work since both phones are GSM. I originally bought my phone outright from Apple, not T-Mobile so it wasn't locked and all that. Should be a matter of swapping out the SIM card and restoring from their backup right? Anyone have a heads-up? Cheers.