Play the dual-tone (DTMF) sounds of a telephone keypad. Press keys or play a whole number. Each key is the classic pair of low and high frequencies.
DTMF (dual-tone multi-frequency) is how a phone keypad signals: each key plays two pure tones at once, one from a low group and one from a high group. Generated live with the Web Audio API; nothing is recorded.
DTMF stands for dual-tone multi-frequency, the signalling system behind touch-tone phones. Each key plays two pure tones at once, one from a low-frequency group and one from a high-frequency group, and the phone system decodes the pair back into the digit.
It plays the same tones a phone keypad makes, which some old systems, intercoms or IVR menus can recognise if held to a microphone, but results vary. It is mainly for learning, testing and nostalgia.
Using two simultaneous frequencies makes each key unambiguous and hard to fake with ordinary speech or background noise, which is why the system has been reliable since the 1960s.