They don't use cell phones (in their cars at least). They have radios, but I doubt they have to use them just for pulling someone over, I would think it's more automated (maybe it's registered as a stop when they look up your plates).
I meant more so on the phone with their friends.. lol.
How often does that actually happen, though? When they're not busy violently breaking up an anti-police-brutality protest, they usually do genuinely care about public safety.