Analog Telephone Circuits (VOIP)

As mentioned, phone technology originally was analog, from start to finish. Analog modulation is the technique used to convert sounds (such as your voice) into an electromagnetic form. The analog circuitry of the POTS telephone transmitter converts the voice patterns coming from the caller’s mouth into continuous electromagnetic signal patterns. These patterns are carried on a telephone line circuit, sometimes called a trunk line, where they are
carried to the terminating end of the circuit. There, analog circuitry converts the signal back into audible sounds so they can be understood by humans.
A good basic illustration of a POTS circuit can be found in an old elementary school science experiment. My fifth-grade science teacher, Ms. Davis, had us punch a hole into the end of two tin cans and connect them using a long string. If we held the string taut, Jodie Schnickmeister could whisper into one can and I could hear her in the other. (I used to love it when Jodie whispered in my ear.)
This simplistic experiment taught the basics of the POTS network: Sound was converted to an analog signal (vibrations) that was carried over the taut string to the receiving can. The string, in turn, vibrated the can and converted the analog signal back into audible sounds.