PSTN stands for Public Switched Telephone Network and is generally known as the traditional phone network. It has been connecting people since the late 19th century, at first over copper telephone lines and later using fibre optic cables, satellites, and undersea cables. All of these are interconnected by switching centres which establish a dedicated connection – or circuit – between callers.

Voice signals can then travel over this circuit. When the caller speaks into his handset, the caller’s phone translates the sound waves into electrical signals. These are transmitted over the PSTN (converted to optical signals for part of the way, so they can travel via fibre optic cables), and the phone at the other end of the line then translates the received signals back into sound.

The arrival of Voice over IP (VoIP) technology – essentially, transmitting voice calls over the Internet rather than the PSTN – challenged traditional telephony and fundamentally changed how people communicate.