I think the cops listen to all cell phone conversations. They probably got a system so they don't got to listen to every conversation but still listen to a big percentage of cell phone conversations.
I remember this guy I was helping at one of my old jobs told me he was retired and used to build certain technology for the police. He also mentioned that the police would listen to cell phone conversations while they ate lunch and would make jokes about how stupid people were for saying dumb ass shit on the phone.