How Telephone Intercom Systems Work

  My building uses the phone lines as the building’s intercom system. I don’t want phone service just to have an intercom. I plugged an old phone in the jack to hear visitors, but I can’t buzz them in. How can I add power to my wired house phone so I can send a tone to the intercom to let visitors in?

This electrical question came from: Dante, a Handyman from White Plains, New York.
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Additional Comments: Great site!

Dave’s Reply:
Thanks for your electrical question Dante.

Telephone Wiring and Intercom Systems

Dante, the phone system that your building uses for the intercom feature is most likely a proprietary phone system that has these unique features that will not be available from your standard phone. Many commercial phone systems such as this us a proprietary central system which requires programming to enable the features and capabilities. Even though the building phone system uses the standard telephone wiring it requires its own compatible equipment to make use of the functions.
There has to be an equipment interface that is in place to “listen” to the phone line and pick up certain tones which will trigger the intercom or buzz someone in and then make the required events happen, such as activate the relay to send power to the (buzz) electric solenoid (door latch), or activate another relay that will patch the phone handset over to the intercom system and away from the phone service in order to make an announcement.
The “system” will be proprietary in that it is a stand alone system apart from the regular phone system. With your phone system you still may be able to use conventional telephone equipment, but you will need to know the codes to activate these other buzz and intercom options, but this “system” makes use of the conventional phone system that you have, or a totally proprietary system – one or the other.

The Following links will assist you with your telephone intercom wiring:

Telephone Wiring Diagram

For more information about Telephone Wiring
Telephone Wiring

Telephone Wiring
This telephone wiring diagram with photos and instructions will show you the direct steps to correctly wiring a telephone jack.