You need to configure both the AppleTalk Remote Access Setup and the Macintosh PowerBook Portable control panel to use the internal modem.
1) In the AppleTalk Remote Access Setup, select the Modem port.
2) From the Apple menu, choose Control Panels and select the Portable
control panel.
3) In the Portable control panel, you have the option to set the serial
port for either the external or internal modem. Set the port for the
internal modem (note that this will disable the external serial modem
port).
Support Information Services