Dial-up and LAN TCP/IP conflict

We have a metro area network with 5 sites. At two of the sites, users that dial-up to the internet receive disruptions on their Lotus Notes, Attachmate, and other TCP/IP apps as the apps try to use the internet connection instead of the LAN, once dialed up. All of the PC’s are NT 4 sp4 with a Novell client. What causes this conflict?