Historically, "Phone Phreaks" used "Blue Boxes" or "Red Boxes" to play recorded or synthesized DTMF tones to trick the telephone exchange into providing free long-distance calls. While modern digital switches have largely mitigated classic phreaking, the principle of signal injection (playing tones into a live mic) remains relevant for social engineering attacks aimed at manipulating IVR systems.
Before DTMF keys became the standard, the world relied on . You may remember the mechanical "clack-clack-clack" of a rotary dial. This system was slow and prone to errors. dtmf key