Try This 1st
First, go to http://www.idir.net/~gromitkc/winmodem.html and confirm that your modem is in fact not a winmodem. If it is a winmodem, good luck. I've been told that some work under linux (linmodems), others do not, but you are probably better off just getting one that you know will work. Also try going to your console and type
setserial -g /dev/ttys* you shoud see results for each serial port showing the UART type, port#, and IRQ. If your computer has 2 built-in serial ports (most do), then check to see if there are jumpers to manually set the port and IRQ on you modem. If there are, set the jumpers to use COM2 and IRQ4 (assuming this will not conflict with another device on your system) reinstall the modem and check that your BIOS understands not to use PnP for that slot and that you do not have a PnP OS. Run Linux and test the modem using the terminal and AT commands, or just query it. This worked for me. But I can't stress enough that the biggest thing is to make sure that your modem is not a winmodem, if it is not, you should be able to use jumpers to set the COM and IRQ. Good Luck.