Matt Blaze put a handheld high-end measurement receiver (Rohde & Schwarz PR100) inside his fridge to test Snowden’s “operational security” put-your-phones-in-the-fridge technique.
“Snowden may have been misinformed. My fridge only provides about 25 to 30dB attenuation (@ 1GHz), insufficient if near a cell site.” https://twitter.com/mattblaze/status/443531362939580416
There’s an interesting trend of CIA-educated people failing the cell phone Faraday cage aptitude test. When the CIA kidnapped a man off the streets of Milan and took him away to be tortured, their operatives thought a potato chip bag would do the trick.
Fortunately for the rest of us, the baggie didn’t, and the Italian authorities were able to reconstruct the whole show.
One question that pops up in my mind. Is the CIA intentionally teaching their people weak technical operational security, so it’s easier to keep tabs on them “in the field” and keep them from “going rogue”?
(where by rogue is defined as what Snowden did, my understanding is a bit of felonious activity or kiddy fiddling or whatever on the side is fine as long as you file a proper report. I am not kidding as much as you may think.)
In any case, some German hackers did the applicable research and found that while neither fridges nor microwaves do the trick, an Ikea stainless-steel double-wall thermos is a most effective cell phone signal blocker. 
The only trap? It doesn’t fit smartphones. Regular phones only!