lsmod to see if ltmodem is there. If it is then check the following threads:

http://www.knoppix.net/forum/viewtopic.php?t=16378

If it is not then do modconf as root and install it by entering through until it says it is installed.
Can't remember where it is but it is somwhere around the misc or extra lines.

Create a symlink to the modem by doing as root:

ln -sf /dev/ttyLT0 /dev/modem

Then choose /dev/modem from the list. At the top tab goto modem and query the modem and if it is filled in with modem info then you should be able to connect. I just went through configuring a lucent/agere modem. I don't know if it will work with all dial up modems but try it and find out.
Hope this helps.

This post has some info for you also:
http://www.knoppix.net/forum/viewtopic.php?t=16396