Re: how about..
Sure, this is a terrible idea for all the prisons in the centre of cities, but it would work really well for Dartmoor.
Mind you, the idea of a Faraday cage round the entire prison is a good one. Not only would it prevent prisoners from using mobiles, but it would also physically stop drones from being able to drop contraband in, and in the event that someone made a hole in the cage, the sudden influx of EM radiation from outside should provide a warning of a breach.
If only it wasn't so impractical to do for an entire prison.