If that doesn't help, then reinstall the Apple Remote Access software.
If you are using a PC Card modem, make sure the modem file for your modem is in the Modem Scripts folder inside the Extensions folder (in the System Folder).
If it is not, check the software that came with your modem.
If you locate a file for your modem, put it in the Modem Scripts folder.
Contact the modem manufacturer to see whether your modem can use the settings for another type of modem.
This document contains information from the PowerBook G3 Series User's Manual.