alt Hacker NewsAirplane mode doesn't actually turn everything off, and it's not a guarantee like a faraday cage is. There are instances of devices still actually transmitting "stuff" in airplane mode, or having airplane mode silently disabled. A faraday cage is absolute, assuming correctly constructed, in that it guarantees zero signal can get in AND out. There's no ambiguity (again, assuming correct construction).
Wouldn’t turning the phone off accomplish the same thing?
All of the bags and boxes others have posted make the phone unusable, so turning it off seems just as well. Plus, if you leave the phone on, without airplane mode, in a faraday cage, it’s going to die rather quickly while it searches for a signal.