The transmitter contains a round metal disk called a diaphragm. When you talk into your telephone, the sound waves of your voice strike the diaphragm and make it vibrate. Depending on the tone of your voice (high pitched or low pitched) the diaphragm vibrates at different speeds this is setting up the telephone to reproduce and send the sounds that it "hears" to the person that you are calling.

Behind the telephones transmitter's diaphragm, there is a small container of carbon grains. When the diaphragm vibrates it puts pressure on the carbon grains and squeezes them closer together. Louder sounds create stronger vibrations that squeeze the carbon grains very tightly. Quieter sounds create weaker vibrations that squeeze the carbon grains more loosely.

An electrical current passes through the carbon grains. The tighter the carbon grains are the more electricity can pass through the carbon, and the looser the carbon grains are the less electricity passes through the carbon. Loud noises make the transmitter's diaphragm vibrate strongly squeezing the carbon grains tightly together and allowing a larger flow of electrical current to pass through the carbon. Soft noises make the transmitter's diaphragm vibrate weakly squeezing the carbon grains loosely together and allowing a smaller flow of electrical current to pass through the carbon.

The electrical current is passed along the telephone wires to the person you are talking to. The electrical current contains the information about the sounds your telephone heard (your conversation) and that will be reproduced in the telephone receiver of the person you are talking to.

The first telephone transmitter aka the first microphone was invented by Emile Berliner in 1876, for Alexander Graham Bell.

The receiver also contains a round metal disk called a diaphragm, and the receiver's diaphragm also vibrates. It vibrates because of two magnets that are attached to the edge of the diaphragm. One of the magnets is a regular magnet that holds the diaphragm at a constant steadiness. The other magnet is an electromagnet that can have a variable magnetic pull.

To simply describe an electromagnet, it is a piece of iron with a wire wrapped around it in a coil. When an electrical current is passed through the wire coil it makes the iron piece become a magnet, and the stronger the electrical current that is passed through the wire coil the stronger the electromagnet becomes. The electromagnet pulls the diaphragm away from the regular magnet. The more electric current, the stronger the electromagnet and that increases the vibration of the receiver's diaphragm.

The receiver's diaphragm acts as a speaker and allows you to hear the conversation of the person calling you.

The sound waves that you create by speaking into a telephone's transmitter are turned into electrical signals that are carried along telephone wires and delivered into the telephone receiver of the person that you have telephoned. The telephone receiver of the person that is listening to you receives those electrical signals, they are used to recreate the sounds of your voice.

Telephone calls are not one-sided, both the people on the telephone call can send and receive a conversation.