I'e had similar problems. I was using my fax and DSL on
the same line for four years and then updated to Windows
XP. I can send, but sometimes cannot receive. Someone
told me that DSL digital frequency fluctuates into the
same frequency as the fax. I was told to get another fax
filter, to buy a better modem, to run a separate line
from the master phone box, that you cannot use DSL and
fax on the same phone line, and a host of other things by
numerous "experts". I serious believe no one knows an
answer.
I know that doesn't help, but misery loves company, right?