[HiTech RedNeck]

Another answer is the Faraday cage. Embed sufficiently fine metal mesh in walls, doors, and windows. This physically contains the signal.

[Bobalu]

Too expensive.
Won’t work.
Does not allow prison employees to use their cell phones.

[_Jim]

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.)