You don't have to always remember which serial port is connected to your modem. An easy way to determine what port your modem is attached to is to create a device called /dev/modem. You can create one from the command line using the ln command.
You can also create the /dev/modem device from the command line by using the ln (link) command. To create /dev/modem if your modem is connected to ttyS0, log in as the root operator and enter the following:
# ln -s /dev/ttyS0 /dev/modem
This command line creates a symbolic link, /dev/modem, which points to the ttyS0 serial port. Check this link with the ls (list directory) command, like so:
# ls -l /dev/modem lrwxrwxrwx 1 root root ...