You used to be able to use a scanner to listen to cell phone conversations decades ago before they were encrypted. It was actually kinda fascinating. The type of 850 mhz band scanner you needed was on the more expensive side at the time, but was still the sort of thing you could by at Radio Shack.

I have a pet theory that there's probably a few people who got rich using insider information they gleaned from picking up conversations in the Wall Street area that the participants thought were confidential.