I read where microwaves made good Faraday cages. Then I did a test. I placed my cell phone inside a microwave and used a second cell phone to call the first. I dialed the number and the thing inside the microwave rang! Well, that shot that idea!
I have a .50BMG ammo can lined with cardboard. I placed the same cell phone inside the ammo can and latched down the lid. Called the phone. No ring.
I also have a few EMP-proof bags that work great.