This is a simple question, I think. Our CSIs use their cell phones to keep in touch, correct? I mean, obviously they do, but that's also what they have and use in, say, an emergency. When Warrick calls an ambulance for Nick in "Stalker," for instance.
I had assumed that, and then realized that Greg uses a radio to call in the attack he's witnessing in "Fannysmackin'."
Maybe the CSIs only carry their cell phone, but the CSI-mobiles (the Denali SUVs) come equipped with police radios as well?