Another answer is the Faraday cage. Embed sufficiently fine metal mesh in walls, doors, and windows. This physically contains the signal.
Too expensive.
Won’t work.
Does not allow prison employees to use their cell phones.
Another answer is the Faraday cage.Really ... REALLY!!??
Where can I find my local 'Faraday cage dealer' then?
(Pssst - RedNeck: Faraday cages per se ONLY block electrostatic/electric fields, NOT magnetic fields and so TECHNICALLY SPEAKING would not work in this case SINCE propagating RF energy contains a balanced E-H field.
You MIGHT be referring to a 'screen room' or screening (shielding) techniques which is what we in this field actually call it. Google Lindgren for a primer on the subject.)