Cell phones work by radio frequency signals that are transmitted from one cell phone to the other via a network of cellular signal towers. It is very simple to make a homemade cell phone jammer, however it is also very illegal to do this in the United States.
British scientist Michael Faraday invented the Faraday cage in 1836. It is an enclosed space lined with electroconductive material such as a brass or aluminium. The metal is grounded to earth so that any electromagnetic signals will be transferred by the metal to ground without affecting anything within the cage.
Build a Cell Phone Faraday Cage
Get two cardboard or wooden boxes. One box should be large enough to hold your cell phone and fit snugly in the other box. Cover the large box with aluminium foil. Attach a wire to the aluminium foil with an alligator clip on the other end. This clip attaches to an electrical socket ground or any metal plumbing fixture. Wrap plastic sheeting around the box to protect the foil from ripping. Leave one end unsealed. Place the cell phone in the small box and the small box in the large box. The aluminium foil will capture any cell phone signals and transfer them to ground via the wire and alligator clip.
An electromagnetic pulse generator used to broadcast interference on any cell phone frequency is illegal. Even if you bought one overseas, use of it in the United States will be prosecuted by the Federal Communications Commission.