I think it would be quite difficult to scan for outgoing
calls from machines on your network.
First of all, you would have to have access to each and
every machine that is on your network. This includes everything
from mainframes to the little desktop PC on everyone's desk.
If your company has all of a very few types of machines and
they are all tightly configured, you might be able to do
it. You would have to have some sort of common software
inventory software running on all of your platforms that
reports back to some central database what it finds on
the machine. Of course, the software would have to be
tamper proof so that someone who wants to dial out cannot
tamper with the inventory software to hide the fact that
they installed modem software.
As I don't know much about the telephone system; I would have
no idea how easy it would be to have something monitor each
of your outgoing trunk lines for modem carriers; distinguish
between outgoing modem calls and incomming modem calls into
your central modem pool. There might be a possibility there.
This would get more cumbersome as your enterprise spans