Re: Ready for the down votes, but...
OK I'll bite...
If only metal shielding stops radio waves can you explain the tremendous amount of Wi-Fi extenders/repeaters on sale?
Prisons tend to have very thick walls and very small windows.
My home femtocell can't get 20 yards past my 9" cavity wall.
Do you really think I would suggest it was done without a site survey!? And 80% coverage means you only need to check the other 20% of cells reducing the burden on staff.
But hey it's only a tech site so let us go with, "I can't see it working so there can be no possible way to implement it!" How very inventive and open minded.
FYI - There were some cheap illegally imported Chinese jammers 10 years ago with a line of site radius of about a 20 yards.