I've had success using winmodem drivers from this site:

http://www.physcip.uni-stuttgart.de/heby/ltmodem/

There are problems with just apt-getting the .deb package because the installation program checks for compatible gcc versions. Knoppix kernel was compiled with gcc major version 2 so the install program considers this to be incompatible & won't install.
The workaround is to download the tarball & follow the installation procedure in the readme. It consists of 3 commands, build_module, ltinst2 & autoload. The first two cause no problems but the ./autload script has the check for gcc version. Despite the dire warnings, it works just fine if you just edit the script so it doesn't do the check. Just find the if statement containing the warning 'but for safety's sake, now exiting' and comment out everything from the "if" to the "fi".