Mobile Phones sometimes don't have
enough 'oomph' in their vibrate mode to
have you notice.
Sometimes their noise alone will alert
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
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.