MSN 6.0 problem connecting to Internet

Cannot connect to internet using MSN 6.0. 1 dial-up line for PC and telephone. My modem is a 56k and it tests fine, with WIN98 modem diagnostics. WIN98 error code 678, when I am trying to connect to internet ( my modem dials fine, but no answer fromMSN server ), it tells me to try again later. I have called MS Tech support numerous times with no resolution, of problem. Only way I can get online is using Internet Explorer 5.5, instead of MSN 6.0. Can you offer any help or assistance? Thanks, inadvance, for your support!