best place to find activated AT&T SIM cards for testing broken phones

HI,

i have been repairing several phones in my spare time, all of the AT&T phones i get in i can't test with my SIM cards i have on hand. i have about 10 AT&T SIM cards from and i don't get any signal out of any of them. Even on an unlocked phone. For example, unlocked Iphone 6, was originally with ATT, i get service to show up in the upper left hand corner of the screen with Tmobile, sprint and verizon. but none of my ATT sim cards show signal. i have tried this on other att phones too. none of them get signal. what are my options? pay for a cheap pre paid plan just to have an activated SIM? i just hate to try and resell without making sure everything is up to speed on these.

Note, all of my other sim cards from other companies are not activated either, but they all show signal when inserted in phones. i am thinking it is an ATT thing?

thanks!