Most dialup modems are not supported by Ubuntu, but drivers can be found that will enable the use of such modems. First you need to identify what chipset your dialup modem is using:
wget -c http://linmodems.technion.ac.il/packages/scanModem.gz
gunzip -c scanModem.gz > scanModem
chmod +x scanModem
sudo ./scanModem
gedit Modem/ModemData.txt
Much more detailed information is available on the community wiki.
