The ATT 8320 does not have the calling over WIFI feature (via UMA/GAN), niether does their network support it. Where as the T-mobile 8320 does (And their network). So unfortunately the calling feature over wifi will not work with ATT. I used to have an 8120 with T-mobile so I used to take advantage of this feature as well
I am looking forward to this feature when I purchase the new 8900 to come out on Feb 11.