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Sysadmin sank IBM mainframe by going one VM too deep

onefang Silver badge

"DTMF had (as the name implies) two tones. Each tone had four frequencies for a total number of combinations of 16. 12 (3x4) were used in the telephone keypad and the other four (ABCD?) did "other things" if you could generate them..."

ABCD is correct, you can buy keypads at electronics shops that have all twelve keys. There's likely apps for that.

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