History
The first telephone-based intercom was patented in 1894 by the Kellogg Switchboard and Supply Company. When a guest pulled a handle on a panel at the entryway of a building it would close a circuit (or relay), allowing an electrical current to pass through wiring to a receiver in the tenant's apartment.
According to Wikipedia https://en.wikipedia.org/wiki/Intercom
An intercom ("intercommunication device") or interphone is a stand-alone voice communications system for use within a building or small collection of buildings, functioning independently of the public telephone network. Intercoms are generally mounted permanently in buildings and vehicles. Intercoms can incorporate connections to public address loudspeaker systems, walkie talkies, telephones, and to other intercom systems. Some intercom systems incorporate control of devices such as signal lights and door latches.
Intercoms are used on a wide variety of properties, from houses that only require one connection between a resident and the property's entrance to multi-unit apartments that require intercom hardware to be installed in every individual apartment. Some are equipped with video, and its wiring (electrical installation), can be connected to the outside with few pairs (4-6 pairs) while controlling an electric strike. The latest generations are even compatible with computers, and some models include TCP/IP compatibility.
In 1912 (possibly earlier), a telephone-style handset or a hands-free desktop loudspeaker was available, then in the late 1930s, the intercoms still took on the appearance of telephones.
Then in the 1950’s intercoms were used to gain entry through the front door, which was electronically locked, you would have to be buzzed in by the person you wanted to see after identifying yourself. You’ve probably seen this in action on any Seinfeld episode where Jerry would have to unlock the door remotely after George call him on the intercom. With the transistor's invention it paved the way for more improvements for the intercom manufacturers by switching over to solid-state circuitry.
In 1982, door-answering devices now included video and audio identification methods.
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