Activating a microwave oven
with a tablet or phone
inside would almost certainly damage the portable electronics
due to the extremely high RF energy density created by the magnetron - 1000 watts inside less than 2 cubic feet.
The Faraday cage has gotten a lot of press in the paranoid community recently, but the reality is that protecting portable electronics
doesn't require it. Just make sure the device has no cables
(charging, headphones, etc.) connected to it during lightning