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 from
lightning doesn't require it. Just make sure the device has no
cables (charging, headphones, etc.) connected to it during
lightning storms.