The various hostage rescue groups have access to cellular blockers, as well as contact numbers at the cellcos, to do various degrees of restricting cellular usage. The main reason is to prevent the hostage takers from being able to talk to other folk, but there are plenty of secondary ones as well.
Oh, and when they _really_ need to block cellular communications, they have been known to physically shut down the local cell towers.
_____________________________________________________ Knowledge may be power, but communications is the key snipped-for-privacy@panix.com [to foil spammers, my address has been double rot-13 encoded]