:D :D :D
So, what does the grounding wire do? And how does wrapping the phone in something non-conductive make the foil more like a faraday cage?
Sorry for all the questions, but I'm always curious about spy stuff in movies. I never know what's real or what's not. In "Enemy of the State", Gene Hackman takes the transmitters from Will Smith's clothes and puts them in a foil potato chip bag. If my home cordless phone was able to function wrapped and sealed in foil, why should I believe that state of the art NSA transmitters wouldn't work when in an unsealed foil bag?
The grounding wire works... well, by grounding.
An antenna works by receiving RF waves, and those waves induce small electrical currents in the metal of the antenna, which the electronics of the device pick up and read. Conversely, an antenna transmits through modulated electricity being run into it, and it gives off RF waves.
Grounding the metal shield of a Faraday cage, be it a purpose built one of copper mesh, or home made from aluminum foil discharges any current the cage may pick up into the ground. Either from received RF from the outside world, that could get through to the device inside, or conversely, from the device inside getting out. It's not 100% necessary because a properly made Faraday cage should allow RF to flow around it, or keep any originating within it inside, (Like the idea of an airliner getting hit by lighting, passes around the plane, through the outer skin, leaving the inside alone) but it makes it more effective, especially if your Faraday cage is not optimal in terms of being complete, or the right materials, or the best possible shape.
Wrapping the electronic device in an insulator, or a confirmed air-gap on all sides prevents electrical contact between the Faraday Cage and the device in inside it. Especially if it has an exposed metallic conductive antenna. Because if that touches the cage, it just effectively becomes part of the device's antenna.
There's also more complicated issues with the size of the wavelength of the RF you're trying to block, in or out, and multiples of that wavelength and the distance between the emitter/detector and the sides of the cage as well. This also has to do with why a Faraday Cage can have "holes" in it, like use copper, or some other kind of mesh, because if the holes are smaller than the wavelength you're blocking, it acts as a solid sheet. (This is why you have the see through mesh in the door of your microwave.)
Most portable consumer electronics no longer have external metallic parts to them anymore, so insulation might not be needed, since the entire case and all exposed surfaces are often non conductive plastics. Although it's possible that even chrome-look plastic trim etc. might have enough metal contact to provide a pathway.
The capacitance/conductive glass of smartphones touch-screens might also be another path too.
Maybe birdman can fill in or correct anything I've said wrong, but that's my understanding of it.