From an article I'm editing on how to install a modem in a computer running Linux:
Edit the file "etc/ppp/options" so it contains the following lines:
debug - log transactions to /var/log/messages
/dev/ttyS1 - ( S1 = COM2, S2 = COM3, S3 = COM4)
115200 - Baud rate. Max supported bu UART chipset, not modem.
modem - Serial link is connected to a modem
crtscts - Use hardware flow control (RTS and CTS) to control serial line
asyncmap 0 - Choice of mapping of control characters
defaultroute - PPP becomes the default route to the internet
noipdefault - Not a static IP address. IP address assigned by ISP.
lock - Don't let other processes besides PPP use device (/var/lock/)
connect "/usr/sbin/chat -v -t 240 -f /etc/ppp/chat-isp"