Re: Wouldn't it be cheaper...
The effectiveness of Faraday cages requires very detailed, careful application (many years ago I worked with TEMPEST kit). I've had my phone in biscuit tins, and in steel filing cabinets, and the thing will still have a signal and ring when a call comes in. Chicken wire's also probably too big a mesh to block a mobile phone signal, due to the ratio of the holes to the wavelength.
My solution is to lock the convicts in steel shipping containers. That wouldn't stop mobile signals - until I'd had the containers dumped at sea.