Leonard Erickson wrote in a message to Roy J. Tellason:

 LE> The full touchtone pad is this:

 LE> [1] [2] [3] [A]
 LE> [4] [5] [6] [B]
 LE> [7] [8] [9] [C]
 LE> [*] [0] [#] [D]

I thought it was something like that,  but it's been a *long* time since I
saw it in some book or other...

 LE> Each row and column has a frequency, so pressing a key generates
 LE> the tone pai for the particular row & column.

Yep.  And the only thing needed for this beyond the stuff you get with a
regular pad is one more column frequency,  the row freqs are the same as
what's already in use.

 LE> And yes, a lot of modems will generate the A, B, C, & D tone pairs,
 LE> just as a number will do v.23. It's just that most don't *document*
 LE> it. 

I'm not surprised,  since that stuff is likely done in software.

FWIW,  I *think* these extra tones are used somewhat in Packet Radio.

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