Here's a fun tidbit. DTMF supports ０１２３４５６７８９＊＃ as you probably know. But it also supports Ａ, Ｂ, Ｃ and Ｄ - not present on modern phones. These tones were added for use with remote computers and such, think today's phone menus.

The lack of them means there's a whole DTMF "column" gone. DTMF works on a grid. When you press a key, the tone for the row and the tone for the column are played simultaneously:

  (Hz) 1209  1336  1477  1633
      -------------------------
  697 |  1  |  2  |  3  |  A  |
      -------------------------
  770 |  4  |  5  |  6  |  B  |
      -------------------------
  852 |  7  |  8  |  9  |  C  |
      -------------------------
  941 |  *  |  0  |  #  |  D  |
      -------------------------

Given the DTMF grid matches the typical layout on dialing pads, I wonder if early DTMF phones would wire the pad in the same grid to generate the corresponding tones?