Dialup connection won't time out

I have a new external COM1 port USRobotics modem which is connected to
a Windows XP where ICS is set up to service Internet to other PC
connected to my LAN. Everything works, but I'd like the modem to
disconnect after 10 minutes of no activity. Under the control panel ,
network connections, dialup, I right-click the icon, properties,
options, and set "Idle time before hanging up" to 10 minutes. But,
the modem remains connected indefinitely. How can I set the modem to
hang up after 10 minutes of no activity? Or, how can I discover which
resident applications are causing the Internet to remain connected?
TIA