There is a how-to at linuxnewbie.org:
Should be a ttyS4 in your /dev directory. If not, see man mknod as I cannot guide you on that one. Mine looks like this:
[rolf@localhost rolf]$ ls -l /dev | grep ttyS4
lrwxrwxrwx 1 root root 10 Jul 14 21:22 cua1 -> /dev/ttyS4
lrwxrwxrwx 1 root root 10 Jul 6 22:37 modem -> /dev/ttyS4
crw-rw---- 1 rolf tty 4, 68 Aug 6 19:14 ttyS4
On another tack, I was able to change my modem from COM5 to COM2 in windows > device manager > com ports, or somewhere around there: it's been a while.