There are plenty of Faraday bags readily available for cell phones.

Look in the digital forensics industry. Field forensic investigators can get bags or boxes (look like Pelican(r) cases), or inserts for Pelican cases (a 1615 fits just right into a sedan's trunk).

Long time ago when mobile forensics was in its infancy they were given out as swag.

The #1 problem is of course that if not in airplane mode, some not too smart phones keep increasing the power to the radio (smarter ones do this for a few minutes then power down radio, then cycle up again). Guess what happens with a bunch of juice dumped into electronics in a locked case inside a trunk in a hot car, with half dozen other phones doing the same thing (because it is never a single burner phone).

In a pinch, 3 to 5 layers of aluminum foil, stainless steel cocktail shaker, ammo can, or combination thereof works.

edit: Yes, if we are discussing this with physicists, RF cannot be blocked, it can be attenuated. The strength of the RF signal is reduced as it travels through different materials, and in theory it can never be completely eliminated. In practicality, the signal only needs to be attenuated until it cannot be picked up sufficiently even when very close by a receiver.

I came here to say what you did. I used to work in three letter agencies and took part in testing faraday bags for clandestine operators. Something about faraday bags that most people don't know is that they have a shorter life than you would think. As they move around and bend, they start to "leak" more RF. WaitWaitWha is also correct that in a pinch, some aluminum foil works pretty well if you're careful. The service will be so bad, that the phone won't likely get packets out or in. Just be thorough when doing it.

Also, I worked with clandestine people and for most of them had threat models more relaxed than a lot of people on HN. What are you all up to???