well probably you have 3 serial ports. two on board and one for your modem. one way to find wich ports are detect is listing /proc/ioports
3f8 states as ttyS0 2f8 ttyS1 3e8 ttyS2 238 ttyS3 that is, if you stick with the normal settings in /etc/serial.conf You can assign any I/O port to any ttySx. Because you board is pci, isa pnp will not interfere. Chek /proc/pci and see what io address your modem is using. If its pci it will be listed. if not listed, its not detected. then, edit /etc/serial.conf accordingly. Even if on start up the module serial.o assigns your modem to ttyS4, you can override this. see the setserial man pages for more details
