To power up a cell phone with electromagnetic induction, you would need to design it with a coil or antenna to receive the external energy and properly direct it to the phone's circuitry. For instance, I use an electronic card key to get in the door at work. The reader gives off an AC electromagnetic field, and the card key is mostly a coil to receive that energy. I seriously doubt if any electromagnetic field would be able to power up a normal cell phone. It could be done, but you'd have to design the phone to do this. Who knows?