I think - though I do not actually own one, so have not actually tried this - that you can get linux drivers for the conextant chipset soft modems from this url:
http://www.mbsi.ca/cnxtlindrv/hsf/changes.html
And follow the links for the driver download.
To be honest though I have always used a hardware modem that plugs into the com ports, they are so much nicer for linux and not so much more expensive.
Hope this helps
Pteren