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Here's what a normal touch-tone keypad looks and sounds like:
1209 1336 1477
Hz Hz Hz
+---+ +---+ +---+
697 Hz | 1 | | 2 | | 3 |
| | |abc| |def|
+---+ +---+ +---+
+---+ +---+ +---+
770 Hz | 4 | | 5 | | 6 |
|ghi| |jkl| |mno|
+---+ +---+ +---+
+---+ +---+ +---+
852 Hz | 7 | | 8 | | 9 |
|prs| |tuv| |wxy|
+---+ +---+ +---+
+---+ +---+ +---+
941 Hz | * | | 0 | | # |
| | | | | |
+---+ +---+ +---+
As explained in the node DTMF (dual-tone multi-frequency), each button produces a sound consisting of two different frequencies, the lower one for the row the button is in, and the higher one for the column. A fourth column of buttons, labelled A, B, C, D (or FO, F, I, P) from top to bottom, had a column frequency of 1633 Hz; these were used to indicate the priority of a call in the Autovon system, but are absent from many phones.
Part of what makes the combinations of tones unlikely to be generated accidentally is that no two of these frequencies produce the simple ratios corresponding to an octave (2:1), perfect fifth (3:2), perfect fourth (4:3), or major third (5:4).
A list of some tunes you can play on a touch-tone phone can be found at http://www.thisisarecording.com/touchtonetunes/. If you want to play music on your phone, do be sure to call someone first—not only so that you have an audience, but also so that your tinny rendition of "Pop Goes the Weasel" doesn't show up on your long-distance bill.
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