I have clients with their ADSL2+ service attached to their fax lines
with no problems observed.
Perhaps the issue is the fax machines attenuators are not set correctly
are are to _loud_ on the PSTN.
In Australia Telstra advised the signal level received at the exchange
should be between -15dB and -17dB. They have a Fax On Line Diagnostic
System (FOLDS) which you can send a transmission to, a report is
returned advising of the quality of your transmission including measured
signal and noise levels.
In old days the fax machine might have a wired jumper block to set the
attenuation, more modern devices would be configured from the front
panel, typically in a maintenance mode. Your good old dial-up modems
with fax capabilities would have an S-Register or two to set the