If you don't have a 355 adapter to connect directly to the serial port, use your modem to dial into the voice mail amd dial **3 at the prompt. This will transfer you to an internal modem. Wait for the screen that prompts you to press enter to connect, then press enter, and follow the prompts from there.
Connect your modem to a station port that is programmed to select intercom first, and have it dial 777,,,**3