In the 80s there was development of communication technology in the public telephone network from analog to digital. This had various positive effects. The signals, for example, became less sensitive to noise. It also became possible to easily modify those signals to reduce transmission errors. This method is called “Time Division Multiplexing” (TDM).
Communication equipment and end devices were still electromechanical and analog for a couple of years. VoIP telephony is only digital, we send only sequence of ones and zeros (data stream). Particular attention at this time is paid to codecs in end devices.
Each VoIP-phone converts the first analog speech from the microphone using A / D -converting the digital stream. Then this stream of data, depending on the codec is compressed, and in the end, is divided into packets that are transmitted via IP-based network.