Most modems require a cable that supports the data terminal ready (DTR) signal. In addition, certain modems require a cable that supports CTS/RTS hardware flow control.
To support DTR, the cable must connect pin 20 on the DB-25 connector to pin 1 on the Mini DIN-8.
To support CTS/RTS, the cable must connect pin 5 on the DB-25 connector to pin 2 on the Mini DIN-8 and pin 4 on the DB-25 connector to pin 1 on the Mini DIN-8.
See your modem vendor or Apple-authorized dealer about obtaining the necessary cable.