Putting a phone into a Faraday bag for security is unnecessary because airplane mode provides the same benefit without the drawback of draining the phone's battery.

“If you're ever tempted to put your phone into one of these shields for any extended period of time, just put it in airplane mode instead. It offers up the same security benefits, that is to say, little to none. But it comes with none of the downsides of putting your phone inside a Faraday cage.”