Scanning networks for dialups

[Danny Cox <dannyc @ gmap . leeds . ac . uk>]

Dear all,

  someone not so long ago (sorry, I've forgotten who) mentioned that they'd
worked with a network somewhere and ran a scan on it to find modems which
were connected.  

  I'd like to be able to do something here, and identify, via a scan of some
description where/when dialups to an external ISP are happening.  I can't
quite get my head around how to do this though .. can anyone help?

Cheers
Danny