An anti-static bag (even the metallized type) probably won't be the best idea. Faraday cages depend on the conductivity of the outer shield to exclude electric fields from the inside, but anti-static bags aren't very conductive (100k-10G ohm square resistivity[1]).

You might even be better off wrapping the phone in aluminium foil, but the problem is then going to be ensuring that all seams in the "cage" are actually conductive.

[1]http://www.esdjournal.com/techpapr/ryne/esdbags.htm

I've recently tried the static bag using a samsung tablet - wifi and bluetooth both showed no attenuation with two different bags. A "farraday cloth" that was suggested to me is priced at about $24 a linear foot (8'x1') so I wouldn't expect these cheaper solutions to be very effective. I would expect a microwave oven cavity to effectively reduce signals in 2.4ghz, though not signals in the lower gsm and related bands.

Tried two different types of anti-static bags -- even if I use two layers, the phone doesn't lose a single bar of signal strength.

I've seen police investigators using some sort of Faraday bags to preserve evidence while working with cell phones.