I'm with Ausatco.
I'm pretty sure you can use fax on an adsl line as in our last office we converted what was a dedicated fax lin into the main ADSL feed for our office.
Dropping the ADSL line sounds likley if you are using a USB ADSL connection but not on a router. Windows may decide it knows best about having two modem conenctions on one line and drop you - not sure, but I can imagine it seeming sensible to a designer.
As for the "speed limit" if my memory is still working this late class 2 fax is sent at 14400 bps if an upper limit of 19200 does apply it will probably have no real impact.
One question Duckbutt, who do you deal with who preffers fax to email? Lawyers?
I have sent two faxes in the last three months and one of those was from my mobile phone as a back up to email.