You need to have your modem reset. If you are running syslogd , debugging info will appear in your log files. Look at the debugging info and see what is going on. Most likely, you will need to tune some of the parameters in your config file, and reconfigure your modem. You could also try mgetty .
Some people have better luck with it.
If it doesn't work at all, then you need to make sure your serial port can be found. Check the BIOS menus and BIOS messages. For the PCI bus use lspci. You may try probing with setserial. See Probing If nothing seems to get thru the port it may be there but have a bad interrupt.