Add an additional intercom station to your Doorbell Fon Door Answering System. If you already own or are planning to buy the Doorbell Fon system, consider adding an additional intercom if you have a multiple entryways to your home or business. Surface mounting makes it quick and easy to instal.This intercom connects to the main control box (included in the Doorbell Fon Answering System) via a single pair wire. The control box has 2 separate connections for a door intercom (D-1 and D-2). You can also connect more than 2 intercoms (up to 4 per input).