For - non-violation jamming of cell-phones (two, one, and zero bar signal responce to cell tower units) the white "magnetic paper" for printers and intended for white boards.... cut into lengths and placed on the metal strips of suspended in-door-room ceilings does a superior job and they blend into the decor. A scattered few on a whiteboard's edges also help. Incoming are not always blocked but the receiver must leave the room to respond.

Sincerely,
Gargoyle

Radio, television and computer (hard wired) are not affected.