Minimal traffic prevents dialup connection automatically closing

We have 60+ XP Pro laptops connecting to our network through dialup modem
connections to synchronize a database on the laptop with the main database on
a server.

Last week, these laptops were members of an NT domain. The Cisco router that
hosts the in-office modems was set to disconnect a dialup session after 15
minutes of inactivity. If a user forgot to disconnect the dialup session,
the router would successfully take care of it after 15 minutes.

This week, the laptops were migrated to a 2003 AD domain and the router
session-timeout was reduced to 10 minutes. However, if a user forgets to
disconnect now, there is a minimal amount of traffic happening on the line,
and thus the router never disconnects the session.

Is there some kind of new traffic between the 2003 DCs and the XP laptops
that wasn't happening with NT? I plan to do some packet capturing on the
laptop to see what I can discover, but I hoped perhaps someone would know a
setting in Group Policy perhaps that could turn off any extraneous traffic
when connected over dialup.

Thank you,
Joshua McConnaughey