Unscheduled calls in an W98 Network

I need to find an utility or some way to find what is causing unscheduled calls in a network with 2 pc?s and a Cisco 76x router. The router is calling to a remote station without apparent reason (the pc?s are left on at night). I have the same settings on another locations with the same equipment and the router doesn?t call. What can be causing this?