alias eth0 8139tooto your /etc/modules.conf (or to /etc/modutils/eth0 on Debian and run update-modules). Unfortunately, the card doesn't seem to support link beat detection, so you cannot use laptop-net to automatically start the network with the appropriate settings (depending on the other machines in the network).
I downloaded the source package for the Conexant HSF modem drivers for Linux from http://www.mbsi.ca/cnxtlindrv (http://www.mbsi.ca/cnxtlindrv). Running make install and then hsfconfig (both as root) installed the kernel modules and set up /dev/ttySHSF0 as /dev/modem. It also made the correct settings to /etc/modules.conf (or rather /etc/modutils/hsf) so the modules are automatically loaded.
The modem seems to work fine now, but I had to comment out the line
#authin the file /etc/ppp/options to be able to connect to my provider Lion.cc at home. This has probably nothing to do with the modem, but rather with the server of my provider.