Steve, it is possible for a cell phone to develope minor magnetic interference that upsets their inbuilt magnetic compasses but this only happens if a magnet it placed directly onto the the cell fone, this wont happen at a distance of 4 meters and i still feel that you are getting electrical interference of some sort....Perhaps you could make an experiment where you eliminate the electric motor from the EM pump and turn the magnets by hand, 4 meters away from your cell phone and see what happens 
