WINDOWS 2000 AND ROUTERS

I have workstations with win2k prof. and a win2k server with this I have a Router to access the internet. The configuration of the TCP/IP protocol of each workstation, I put the IP address of the router for the gateway and for the DNS, the IP address of the internet service provider. It works fine when I need to enter the internet but the router is always connecting to the internet spending a phone call out need, each time a computer is turned on or when accessing other stations in the network.Where am I doing it wrong? Is there a configuration that I have missed? Please do help me, it is most urgent!