Back in the day with the Nextel phones and service the two way walkie talkie bit use to be impossible to monitor. The FBI made them basically break the security on it. Then right inside their data center they had a rack of their equipment. For engineers there was no drama in listening in to any phone call (press a button basically). Now that all this has come out, I would not be the least bit surprised if the FBI just pressed that button without a warrant. They definitely didn't need physical access.