I wouldn't even bother calling. Just b.c it is a GSM phone does not mean they will let you do it. The ATT phone has to be activated on ATT and so on etc.
I am sure their "system" prevents them from activating the ATT phone with a T-mobile number. It is all done with a scanner and nothing to do with the SIM in the phone.
I am sure their "system" prevents them from activating the ATT phone with a T-mobile number. It is all done with a scanner and nothing to do with the SIM in the phone.