I used to have the old AT&T unlimited plan. A few years ago, they cancelled it due to tethering. I called in to complain, as I know how to tether “illegally”, but hadn’t been doing it. Eventually realized that I’d connected my phone to my mother in-laws new Jeep Grand Cherokee, and the Jeep must have been making IP connections through my phone. Just to spite AT&T, I switched to Verizon.