This page describes the intercom system.

Intercom systems are well known as closed telephone systems with a variety of functions.

When looking at the entire system, the intercom system can be described as a huge matrix system with many inputs and outputs. It may give you a different impression than just a closed phone call.
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The advantage of the handsfree intercom is that it can be used not only as a well-known telephone-like calling, but also as a small personal SP/MIC around the desk-scale.

Each call is completely independent from other calls. This means that one call does not affect another call. In the image above, images of two people are used, but please assume that they are all different people. Many of them can make independent calls at the same time. The intercom system is essentially a huge matrix. And it's like a luxurious PA system where each person is given a microphone that can be used hands-free and a speaker that only emits sound within their personal range.

It is also a big feature that it is broadcast from the station using a speaker. Some intercom stations have SP connector. The station is also a small AMP that can be broadcast only in a small area. These speakers are not bound to one line, they are independent lines with AMP in each.

A PA system can forcefully transmit sound over a wide area. If the target audience is clear, the broadcast area is narrow, and the number of simultaneous broadcasts is large, users may find it more comfortable to use an intercom system instead of a PA system.​