I work at an EMI laboratory with four shielded rooms. I can tell you that it is *extremely* difficult to effectively shield an enclosure from radio waves, especially in the gigahertz range (where cell phone frequencies lie). FM radio signals, with much larger wavelengths (and thus far easier to block), can penetrate the room if we get a small crack in one corner. Also, any wires that penetrate the walls can carry the signals via conduction - think power lines, cable TV, POTS phone lines, etc. This is a noble effort, but the chances of it being highly effective are low.