Loopback Detected on a Modem?

Attempting to use RAS on an NT Workstation 4.0. Receive an Error 737: Loopback Detected when DUN is activated. Up until yesterday this client worked fine. TechNet list Error 737 as a valid error message for RAS. But no clue as to how to fix the problem. I have uninstalled RAS and reinstalled same. Checked phone lines for degradation, none apparent. Any suggestions???