Every click sends two tones on two different frequencies. Number 1 consists of the tones 697 and 1209 Hz for example. DTMF came to the US in the 1940s as a streamline of a system for telephony that then was built on the impulse system, with a rotary dial. The technique isn't interruption sensitive and is considered reliable. The US based company Bell Labs invented the technique.
Short versions of DTMF tones as MP3 files to download and to play.