Maybe try checking that all the fax settings are setup correctly. Is there an answering machine in line with the 6605? That can really disrupt receiving a fax.
To step through all the fax settings, go to the documention page for the 6605: http://www.support.xerox.com/support/workcentre-6605/documentation/enus.html
Download the WorkCentre 6605 User Guide and navigate to "Configuring Fax Defaults and Settings" on page 70 of the pdf. There could be the slightest setting that is causing the error code.
Regards,
Seth