I haven't done this yet, but I need to one one system. I have done it on lots of SYSV Unix. You need to run some form of a 'getty' daemon on the serial port of modem. Have a look at SME DOC: /usr/share/doc/util-linux-2.11n/README.modems-with-agetty
I think that is the right program, and I'll soon find out.