Dialup Networking

In our shops we use win xp pro on the desktop. The connection made to the server is made via router and isdn dialup. The email checks every 30 minutes. And the people consult the server via browser. They can only surf on intranet.
In the formerconfigurations (win nt), the line was closed after a minute of inactivity.

Now with XP the line is constantly opened/reopened and our telecombill is extremely high.

Are there registry keys or other settings to keep the bill a little lower ?thx.
webwoefel.