The disadvantage is that I have to have reception. Ironically the worst place for reception is in the apartment. Also, quite often either I or my flat mate are abroad so if you phone one of us then it might well be silly-o-clock where we are.
So what I want is use something like Grand Central or maybe an install of Asterisk or Adhearsion set up with a phone number that calls multiple mobile phones when we're buzzed.
However, there could also be a motion detector (or something that detected our phones connecting to the wireless) in the apartment so that it would also add the land line.
Furthermore it could check our Dopplr feeds and work out if either of us were out of the country and then remove them from the "to call" list.
Because, you know, having a door entry system you have to debug rather than just fricking work sounds like a wonderful idea.