If it is a hardware modem, just try out the various serial ports in your dialing program and see if you can find it (com1=/dev/ttyS0, com2=/dev/ttyS1, ...). If it is a winmodem (it prob. is), you will need to get the drivers for it if there are any. See
www.linmodems.org . There is a tool there for checking if there's a driver for your modem.