I made a bet once, claiming that a cellphone wouldn't work in a microwave (obviously, with the microwave not operating) because it's a Faraday cage. We'd get a call going, put the phone in the microwave and close the hatch. See if the call drops. The tech back then was 900Mhz GSM.
I lost that bet.
While we were at it, we tried all sorts of metal containers one can find in a household, and couldn't find anything that would make that GSM call drop.
Not taking any position on the actual effectiveness of Faraday cages, but I'm pretty sure cell phones can handle a order of magnitude or two (10-20dB) drop in signal strength on a good day, especially if your Faraday cage is also blocking all the background noise the signal would usually be competing with.
I lost that bet.
While we were at it, we tried all sorts of metal containers one can find in a household, and couldn't find anything that would make that GSM call drop.