If TF still allows activation of a BYOP AT&T SIM, you can do it then use the SIM unofficially in a TF-branded phone. It doesn't matter whether or not the phone is unlocked by TF, its IMEI is still paired to its original TF/VZ SIM. Unlocking by TF allows the phone to be used with a non-TF SIM, but does not change how it behaves within TF system.
Bookmarks