If you are getting a Page Not Found Error when you try to access the Impact Fax Server via http://localhost:port then either one of three issues exist:
1) The Impact Fax Server Service is not running - restart the service
2) A firewall is blocking the ports. Open up ports 1171 1172 and 1173 in your firewall. Windows 2003 requires that you add the server as a "Trusted Site" it also has a similar firewall to Windows XP so make sure you open the proper ports.
3) You have a network configuration issue that is not allowing you to resolve the localhost:port address. Try typing in the server's actual IP address instead i.e. http://127.0.0.1:PORT (using the actual IP numbers for your server). There is a Microsoft Knowledgebase article on this network configuration issue for more information see article 290766 at http://support.microsoft.com