Why phone keypad tones are two notes at once

In the push-button standard, every key is two notes at once: one low, one high. A 1960 Bell paper explains why. A single
simple tone is easy for speech to imitate, and the danger was your vowels. Seven of today’s eight notes are the ones
chosen in 1960.
Full video: How does a phone call find you? (link in the related-video slot)
Sources: L. Schenker, “Pushbutton Calling with a Two-Group Voice-Frequency Code”, Bell System Technical Journal 39:1,
Jan 1960, pp. 236-244; ITU-T Recommendation Q.23 (1988), §5-6 and Figure 1 (redrawn). Our own comparison of the 1960
table (p. 243) and Q.23 §6 gives 7 of 8 shared notes (1094 Hz in 1960 only, 1633 Hz in Q.23 only). Your phone’s own
key beep may differ. Sounds recreated for illustration.
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