If you are installing a modem, see http://start.at/modem first to make sure the modem you are going to use is a real modem and that it will work in your machine, under Linux.
If you get the right type of modem, Kudzu may configure it automatically for you, in which case, the serial port will be set up with the correct line speed and UART type and /dev/modem will be linked to the port.
If Kudzu doesn't set up the modem, the procedure for configuring a PCI modem is as follows. First, you need to find the IRQ and I/O address of the device from the /proc/pci file and then use setserial to configure the serial port:
cat /proc/pci
Bus 0, device 10, function 0:
Communication controller: PCI device 151f:0000
(TOPIC SEMICONDUCTOR Corp) (rev 0).
IRQ 12.
I/O at 0xbc00 [0xbc07].