Most people go out an buy an external modem (never a WinModem) so it will work with Linux. You just hook it up to your external Serial connection and go. These modems are more expensive, but that is becuase they have CPUs and in some cases memory in the unit. That is what I did. I have a USRobotics v90 external modem for such use.
Do a search for Linmodem on the web or in Google. There is a project to get WinModems working in Linux, but I think it is still in its early stages.
"The eyes of the LORD are in every place, Watching the evil and the good." Proverbs 15:3