The telephone is divided into two parts: the part that sends the voice and the part that receives the voice. The sending part converts the voice into electricity, while the receiving part converts the electricity back into voice.
The voice-sending part contains a carbon particle with a thin steel plate and an electromagnet attached to it. The voice first shakes the steel plate inside, and the shaking of the plate is transmitted to the carbon particles. When the voice is strong, the grains of carbon are suppressed, and when the voice is weak, they are loosened. When the grains are held down, more electricity flows, and when they are loosened, less electricity flows. In this way, the voice is transmitted through the wire as a signal of electric strength.
The receiving part also contains a thin steel plate and a magnet. The strength of the electricity that is transmitted causes the steel plate to shake and the voice is restored. The voice we hear over the telephone is not the real voice of the other party, but is in fact the sound of the steel plates in the telephone shaking.
The next most important thing is the switchboard. This is a machine in a telephone station that allows us to make calls using telephone numbers.
When you select a number by dialing or pressing a button, the switchboard at the telephone station first connects the call to a switchboard near where the other party lives. After that, the call is connected to the other party's home phone.