I have had it happen to me! I had a newbies plane secured and was running in the engine for him. I had the engine at an idle when the new guys friend walked up to watch. When he came within about 5 feet of us, the throttle started jumping all over the place. I was watching the engine rev up and down trying to figure out what was happening when I looked over and saw the guy had a cell phone. I asked him if it was on and he said that it was. I had him step back a few feet and the interference quit. I don't know what type or brand of cell phone he had and don't know if it makes any difference but our new club rule is ". No cell phones within 20 feet of any aircraft!"[8D]