Seems to act in a similar way to callback except you don't have to get RTNR or Busy before you can use the feature.
So you need to contact somebody with a message?
Well if your phone is enabled (ADC) and the person you wish to contact has the Manual Message Waiting Key programmed, you simply key in *31*ext# and this will activate their light (MMW key), they then press this button and the call goes through to the extension that requested MMW.. Call is established and the message is passed verbally!!
Worked it out myself, but thought i would post anyway.. could be a useful little feature.