Mobile Phones sometimes don't have enough 'oomph' in their vibrate mode to have you notice.
Sometimes their noise alone will alert you, but the point of 'vibrate mode' is to be quiet and feel the alert.
I suggest that a phone be created that includes a small cylinder in which a piece of heavy ferrous material resides. An electromagnet throws the piece of metal to either end of the tube every other second or so. This would in effect 'shake' the phone from side to side or up and down.-- crazyrog17, Apr 10 2008random, halfbakery