I've been hunting for this for a long time. I've been able to find "Faraday" bags that (allegedly) block signals to/from the surveillance unit (phone), but they never mention anything about soundproofness, which is just as important IMO.
Let's say somebody puts their phone into one of those "Faraday bags" and they talk with their friend about private matters. While the phone cannot directly send this recorded audio back home, nothing stops it from doing it the very moment it again is able to send and receive signals.
Also, any built-in hardware would be able to record the movements of the phone the coordinates home, perhaps with just a little worse accuracy than if it had the cellphone towers' signals to compare to in real time.
I mostly worry about the audio being recorded, though, yet none of those bags even mention sound at all. It seems pointless to buy such a thing that just stops "direct"/realtime signals, but does nothing to prevent (slightly lagged) data leaks.
Is there a "cellphone securer" bag that I can buy, so that I can be around somebody who insists on having a phone with them at all times, so that they in theory could stop, take it out from the bag, and use it to make an emergency call or whatever?