[Ol' Dan Tucker]

A tightly closed metal box will greatly attenuate electromagentic (EM) waves from coming in the box and prevent EM waves from the leaving the box. EM waves reflect from metal objects (radar)

I decided to try a test. I put my cell phone in an empty ammo can and then called it.

You're right. Ammo cans will prevent cell phones from sending/receiving signals.

[Bruce Kurtz]

aluminum foil works too, try it.