If you want a real fun thought exercise, think on this. Unlike a mechanical switch, cell phones are activated by an electrical signal. While most of the time that signal comes from the keypad, it's doesn't have to be that way all of the time.
I’ve thrown this out at parties to get a rise out of people.
Techies can turn on fones surreptiously without the device signaling anything and/or an active one can be used as a mike without the knowledge of the owner/operator of the fone.
Gasps of disbelief, and you’re a nut follow.
Fine, a nut, but doesn’t mean I’m incorrect.
Anyone possessing a functional cell phone has approximately zero security. Faraday cage might marginally improve that assessment.
There are ways to avoid giving away a transmitter. Not foolproof.