When I was in high school a friend and I bought a portable scanner from Radio Shack to do exactly this after reading an article about it in 2600.

After getting bored chatting with people on our BBS or hacking around in Turbo C, we'd cruise the suburbs and listen in on phone calls.

We never stopped the car, so it was just a leisurely drift over usually mundane conversations. When you got a handset instead of a base station (or maybe the other way around, I forget), you'd only hear one side of the conversation, and it was fun imagining what the other person was saying.

Magical times.