Configure the modem to use an I/O port and IRQ that don't conflict with anything else in the system.
The devc-ser8250 driver should autodetect the modem, and it should appear in the /dev directory as serx, where x is an integer.
Entries will usually appear in this fashion:
Comm1 is enabled in the BIOS Comm2 is disabled Modem is configured to Comm2's ioport and IRQ
In the /dev directory you'll see: