Yep guards smuggle the phones in or- get this- recently they caught someone using a homemade device like a potato cannon to shoot phones, well padded presumably, over the fence!
MMMMM Jalapeno poppers....AAAAHHHHHHHHI hope you never have an allergic reaction to shrimp jalapeno poppers and need an ambulance! :laughing:
I think, as a number of others have suggested, the answer is a "Faraday Cage". Whole buildings can be Faraday cages, some of them are but not by design. We call one of our company's buildings the "cone of silence building".
I doubt that making buildings faraday cages violates any laws like a jammer would.