The fax service requires a normal dial up modem as the signal used is the same os for normal voice communications. ADSL or Broadband is dedicated to the transmission of data at a very much higher frequency than that used for voice comms. The result of this is that a fax machine would not be able to process or even recognise the signal over a broadband connection.
The bottom line is that you still need a normal dial up modem (56K) to send or recieve a fax.
Why has this not been built into ADSL modems? Well you may ask, but to do so you would basically be building in the exact same dial up modem but at a much higher cost. With such modems available for as little as £8.00 why would you want to add more cost (and complexity) to an ADSL modem?