If more than one IO address is shown, the first one is more likely to be it. See What is Setserial for more info. See the next subsection All Modems for further instructions on quick installation. It can set them into the PnP BIOS's flash memory where they will be used to configure for Linux as well as Windows. For Windows3.x you can do the same thing using the ICU under Windows 3.x.
All Modems Plug the modem into a telephone line.
Set the serial port speed to a baud rate a few times higher than the bit rate of your modem. Minicom is the easiest to set up and to use to test your modem. But if you are lucky you may get ppp to work the first time and not need to bother with minicom.