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.
In 2004, I was doing something similar on Tmobile. Flip phones internet was cheap. Could tether it via a cable to laptop and I also used an external antenna to improve signal. I would leave it up overnight downloading videos on p2p. I was in Army barracks at time and no one else had decent internet.
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.