The internal modem connector shares the connection with one serial port. In case of a PowerBook 180 with two separate external serial ports you have to choose to either keep both external serial ports operational or to use the internal modem. To use the internal modem renders one external serial port useless.
In a PB 520 with just one external serial port one has to choose between using it as a Modem/Printer port or as a LocalTalk port. The second serial port lives hidden inside the PowerBook and waits for a modem card to become useful. As the serial port of this machine has a tendency to die sooner than the rest of the machine it would be a nice hack to make the serial port connector for the internal modem available as a standard serial port.