I'm pretty sure, kg, that you will need an adsl capable fax machine which I think
is available (at a price) since I think the whole machine fax system needs an ordinary voice line to work, but I think it will probably be cheaper to keep the extra line! I work as Keef with 'winfaxpro' with no problems on an ordinary dial-up modem. I am away from the desktop at the moment but I recall there ?may be? some add-on in Winfax to do with b/band which I have not explored, but I do not have the programme here to check. I don't think even the VOIP lot have got there yet, although I guess it is only time.
http://www.btofaq.net/articles/misc/internet2fax.html seems to agree.
ATB