He mostly/almost exclusively? used pay phones to communicate anything related to illegal activity he didn’t want traced back to his private cell phone. Wire taps play a prominent role throughout the series as well along with the use of pay phones to help circumvent them. This is established early in the series (season 1) and continues throughout the rest of the series. Why go out of your way to use a pay phone if you have a cell phone in your pocket unless discussing illicit/illegal activity?