Protocols and standards
DTMF
Acronym: DTMF
DTMF is tone dialling: each key emits two simultaneous tones that the network interprets as a digit.
It replaced pulse dialling and is still used, well beyond dialling itself, to navigate voice menus. Its nature makes it fragile on a packet network: those tones are sound, and a codec designed for speech can distort them to the point where a digit is lost or duplicated.