For all you smart Alecs, it's a lot harder to build a Faraday cage which is effective at blocking mobile phones (900MHz/1800MHz) than you think. If you're in an area with a reasonable signal strength, a mobile will often still work when shut inside a (non-operating) microwave oven... (many dB attenuation, but still enough gets in/out). You'd need a mesh that's finer than your average prison bars, and lots of gold-plated fingers around all the door seals. Would cost a lot of money, and wouldn't last long in a prison environment I'd guess.