One late night many decades ago, I chanced upon a technical description of the Touch-Tone system. The book I was reading had an explanation of how each key on a telephone sends a combination of two to
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What is DTMF? How Does Dual-Tone Multi-Frequency Work? - United World Telecom
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In-Band Signaling: Dual-Tone Multifrequency Dialing
In-Band Signaling: Dual-Tone Multifrequency Dialing
What is DTMF (dual tone multi-frequency) and how does it work?
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