I'm sure that they have all sorts of methods. On the other hand, cellphones make devilishly difficult "bugs" to eliminate, especially the ones that are capable of automatically answering the call and activating the microphone without any audible ring. You can't just block all cellphones, because many people carry pagers that work on the same frequencies, and many people carry cellphones that they depend on.
You can, and they do. Most of the secure government facilities I've been into have a strict policy that absolutely nothing that can or does output a signal is permitted into the building. That means cellphones, two-way pagers, you name it. The official reason I was told was that such a device may 'accidentally' transmit classified information out of the facility. But I suspect the real reason is so that any signal they detect that they don't know about can be treated as hostile. DS