You do not need to come off of broadband to use the line for Fax.
Our office uses the line the ADSL is carried over as its fax line with no problem at all.
Assuming you are on ADSL as opposed to ISDN of course!
However I am not aware of an ADSL "modem" that lets you use it as a fax modem too, ADSL uses a completley different set of frequencies to a standard data / fax modem.
So just connect the 56k modem / fax modem to the PC as usual and then into to your adsl line via a splitter as if it were a telephone, you may need a double socket to share the splitter with the phone.
As CAPI means COMMON-ISDN-API could you perhaps be trying to install some ISDN fax software as opposed to conventional fax software? - WARNING I read no further than the definition of CAPI on the page so that is a guess! If that is not the case google shows up serveal sets of CAPI drivers. Search for the manufacturer of your modsem and CAPI driver.
Good luck! or try fax to email and
www.tpc.int !