Stopping Illegal Prison Cell Phone Use

Locate four cell towers, 200 yards out from the boundaries of the prison, staking out the roughly four corners. Have all four listen for transmissions. By measuring the slight delay between towers against internal atomic clocks, triangulate and locate the offending phones.

You are not listening to the conversation, no privacy laws broken, you are merely tracking down the location of the illegal phones, which a special squad, not local to the prison, would then search for, via video or in person. These towers could be portables, moved from prison to prison, to save money.