IMHO there is only one possibility to send faxes over ISDN: via CAPI. You need to enable CAPI-Support in your kernel (on RedHat mostly available) and install the CAPI-Driver. CAPI-Drivers are available from SUSE for AVM Fritz (passive) and the other AVM active cards. I think that there were no drivers for USB-Modems.
Buy a low-cost serial faxmodem and begin faxing. That may be the easiest way to fax under linux.
I hope that these lines helped a little.