Aluminum foil blocks cell phone signals because of the Faraday Cage Principle. Micheal Faraday, a scientist from the early 1800s, discovered that an electric charge exists on the outside of a charged conductor, and the electric charge has no effect on anything enclosed by the conductor. This is so because the electric charges on the exterior cause internal electric fields to cancel.
Wrapping a cell phone in aluminum foil creates a Faraday cage. Because cell phone signals are electronic, the aluminum foil prevents the signal from reaching the cell phone.
Certain types of metal are often used in the construction of buildings with areas where cell phone use is not allowed, such as hospitals. This creates a sort of Faraday cage, preventing cell phone signals and other electronic impulses from reaching these areas.
The Faraday Cage Principle can be used to either contain or exclude electronic fields and impulses from a room or area of a building. This in turn protects sensitive electronic equipment in places such as hospitals, clinics and scientific laboratories or research centers.