Re: Ready for the down votes, but...
"Yeah sure that'll work, because everyone knows you can make radio waves stop at a specific distance without having to resort to any sort of metal shielding."
A valid point however realistically I dont think it a major issue.
Ok we have prisons with homes build basically right up to the fence in some areas (Norwich for examle) and in some cases we might not be able to implement this, however for the majority of prisons not in built up areas a few low powered jammers dotted around the prison buildings would suffice.
Of course, the real way to tackle this would be to stop the phones getting there in the first place... but thats probably harder to achieve.