It's true that you need a fax capable modem. However, I found that there may
be a catch 22, in that some multifunction devices expect to have the windows
fax system installed before the multifunction unit is installed.
Basically, you can cheat to some extent by installing a standard external
fax modem to a serial port, then install the windows fax system. (even if
the external fax modem is not connected with a multifunction device.)
In your case, a standard external fax modem will connect to the ADSL phone
line thru a filter, just as a standard analog phone might. I used a USR V.92
external fax modem to get windows fax to load and setup. Then changed to a
Brother 720CN connected via a home network for actual use.