Assuming that its a hardware modem (read non-WinModem) and you've got it on a standard Irq and base address then you will just be able to access it as /dev/Stty0 (if com1) thru /dev/Stty3 (if com4). Its not on a standard IRQ/base address you might need to play around a little. Read
for more details. My experience in the past has been its easier to move things around and get the modem on a stardard IRQ & base address, when I first got my machine running I spent hours getting a modem working on non-stardard ranges, mind you that was with the 2.0.x kernels, things are probably a little friendlier now.
If you suspect it to be a WinModem have a look at http://www.linmodems.org/
I can't say I've ever needed to but that will be the place for the latest info...
PS There was a thread today that mentioned what a winmodem was if you're not sure...