No Direct connect to LAN after dialing i

My W2K Pro PC is on a corporate IP network. Software such as Lotus Notes works fine, until I use a Dial Up connection to my ISP. Once I connect to the ISP, the software returns errors that the server(s) are unavailable. Doing a Tracert on a local server name shows the traffic going to the DUN. Is it not possible to have both a direct connection to one network while having another dial up connection to an ISP? What’s wrong here?