Many products promise to completely isolate phones from the rest of the electromagnetic world—Faraday cages for your gadgets. As Matt Blaze points out, modern cellphones rarely stop talking completely, even when they're supposedly turned off or in "Airplane" mode. He put three commercial pouches and three DIY ones to the test.
At least in the frequency ranges I tested, two commercial Faraday pouches (the EDEC OffGrid and Mission Darkness Window pouches) yielded excellent performance sufficient to provide assurance of signal isolation under most real-world circumstances. None of the makeshift solutions consistently did nearly as well, although aluminum foil can, under ideal circumstances (that are difficult to replicate) sometimes provide comparable levels of attenuation.
Turns out a cookie tin won't do.
I have the Mission Darkness Non-Window [Amazon] and presume it can't be worse than the windowed model Matt recommends. It's got a nice unobtrusive style and is very well-organized, but is still… big, compared to a normal wallet.