I believe you are thinking of a
Faraday Cage. I suspect you thinking of the following
security use:
a Faraday cage can prevent the escape of the EM fields emitted by a cathode-ray-tube (CRT) computer monitor. Such fields can be intercepted and translated to allow hackers to remotely view on-screen data in real time without the need for wires, cables, or cameras. This practice, known as van Eck phreaking, can also be used by government officials to view the computer activities of known criminals and certain criminal suspects.
Thus, a properly constructed Faraday cage could prevent the government or others from conducting van Eck phreaking.
Faraday cages can also block outgoing and incoming cell phone calls and radio signals if they are
constructed properly:
The effectiveness of a Faraday cage or shield is dependent upon the wavelength of the electric or electromagnetic fields it is intended to shield. This explains why a microwave oven, for example, can perform such shielding from the observer peering through the metal mesh screened "window" at the front of the oven to watch the cooking process take place. The holes are sized such that the waves within the oven cannot pass through even though visible light which has a much shorter wavelength easily passes through the holes. This also explains how cell phones have improved in building performance using the higher frequencies (shorter wavelengths) of EMFs than the earlier predecessors, notwithstanding improved digital modulation algorithms in so called 3G handsets today and later standards forthcoming. Quality levels of shielding also depend upon the types of metals used in the cages as well as the thicknesses.
Thus, a properly constructed Faraday cage could prevent people inside from making, or receiving, unauthorized telephone calls or radio communications.