If you've got a Rockwell or Conexant chipset based modem and you can't seem to get it to work with Linux, then you've come to the right place. Chances are that you've got a Software modem (also known as a "winmodem") and you'll need a driver to get it working under Linux.
If you've got a Software modem whose chipset is not made by Conexant or Rockwell then you should see the linmodem howto. This guide contains information about both Conexant and Rockwell chipsets as they are both in many cases the same product, this is due to Rockwell splitting off it's chipset manufacturing group into Conexant.
Note that this guide has no information about using Conexant/Rockwell modems for operating systems other then Linux except the for the following: There are no known drivers for FreeBSD or BeOS, Windows drivers can be obtained from Conexant.