I remember back in the 00's on AT&T getting an unlimited data plan addon for a dumb phone was like $15/mo or something while adding it for a smartphone was like $40 or more. They would enforce it by checking your IMEI and seeing if it was one of the smartphones they sold.

Buying an unlocked phone of a model AT&T didn't sell seemed to never trigger the "you're using a smartphone" check. Fun times with some cheap 3G back in the day.