what kind of modem do you currently running under windows.
if it is a winmodem, it will not work unless you download driver for it.
my suggestion is for you to buy a real modem such as, 3com USRobotics.
fisrt of all, you need to go to redhat website to see what modem they are supported.
I used to run redhat as well, and my modem was a 3com USRobotics 5610A
you can try this command on your xterminal
look for the name of your modem; if you see it, it probably a real modem
the price for a real modem is around $79 to $ 100
if you have a real modem, you can email me back and I will tell you how to set it up