Modem disconnect after timeout Win95/98

We are planning to work with NT 4.0 Terminal Services. One of our problems is that a Win95/98 workstation with DUN/modem connection to the server does not disconnect after a idle period.
Even when we do nothing there is still some traffic on the line which keep the line up . If we cannot handle this problem our telephonecosts will be sky high. ADSL is no option . Can you help us ?