You may be better off asking this in a more general-purpose Windows printing
and fax group, e.g. microsoft.public.windowsxp.print_fax, especially if the
problem occurs whatever type of document you are trying to fax.

FWIW, if I were experiencing this problem, I would probably
a. check that I did not have any non-standard modem initialisation string
(unusal these days)
b. have a look at control panel|add/remove software to see if Windows
Update/Microsoft Update had downloaded any updates to my modem driver
c. consider uninstalling/reinstalling my modem driver, but only if I knew I
had the correct driver available and  after doing a system checkpoint.

but that's about all I can suggest).

Peter Jamieson