dial up internet options

When I'm at home I want my internet connection to kick me off if the phone rings (it's not doing that now) however if I'm downloading or uploading files I want to disable this so that the internet will take precedent over an incoming phone call. How can I turn this feature on and off? or where do I set this property so I create a connection that kicks me off and one that doesn't?

afaik, you need a special line, becauase to an outside caller you are just engaged, so the exchange cannot tell if you are on the net or talknig to someone else....