Jamming Cell Phones

Engadget has mentions a piece of equipment that are becoming all the rage at churches. Yeah, cell phone jammers. Tired of the people who can't seem to figure out how to set their phone to vibrate? No problem, just cut off all their calls! $2000 gives the quiet. Of course, what about the docs, volunteer firefighters, etc.? Our church has someone sitting at the phone each Sunday morning. How tough is that?