can at&t detect other carrier smart phone?

view story

http://forum.xda-developers.com – Hello, I got a unmodify unlock tmobile samsung galaxy s2 and is currently using it on att. I just got a email and text message from att saying they have detected it as a smart phone. Even though it only get 3g data, how did they know it is a smart phone? Do they have access to look in to my google play account or my amazon account app account to see the phone model? I am asking because the Tmobile IMEI is not in ATT IMEI file. (HowTos)