Will work with a post-paid AT&T SIM right away- as long as Best Buy has notified AT&T of the sale. If not, please send a copy of your receipt, IMEI and your tel# to firstname.lastname@example.org or email@example.com. Otherwise, the phone may appear to be working but in fact doesn't display the LTE logo on top (i.e. no cellular data), and outgoing calls would be directed to AT&T device restriction department.
The phone can be UNLOCKED by AT&T AFTER 6 months of AT&T prepaid service for sure, hopefully after 6 months of AT&T postpaid too.