The telephone is a great invention. For many ordinary users, making a phone call is commonplace. When two people make a phone call, they simply connect the two ends with a single line and they can make a call. Then they choose to use a telephone exchange. Excess?

In fact, the principle of the call line is not so simple. The above idea only considers two users talking. It is completely feasible to use two lines to connect directly when two users talk. (Figure 1.1 is a special phone for two users to pass through. Schematic), but if there are not two call users, but three or more, then one line will not allow more people to talk.