Touch Tones:

There are 16 touch tones.  Most phones can only produce twelve of these,
the familiar  0 through 9, #, and * tones.  There are, however, four other
touch tones, usually designated A, B, C, and D.  Some modems and special
purpose phones can produce these, but in general they are seldom
encountered.  The frequencies used for each touch tone are listed in the
table below.

In North America, the minimum touch tone length is 50 milliseconds; the
minimum separation between touch tones is also 50 milliseconds.  Touch
tones must follow these conventions to be processed accurately by the DTMF
Decoder application.

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

Dial Tone:

The standard dial tone is a continuous signal composed of equal amplitude
350 and 440 hz tones.

Rings:

There are two common types of rings, normal and PBX. The normal ring is
a tone containing an equal amplitude mix of 440 and 480 hz tones.
It is on for two seconds, and off for four.
The PBX ring is spectrally identical to the normal ring, but is on for one
second, and off for three.

The ring sound is just another DTMF tone,
The difference frequency produced by the mixing of the
two nearly equal frequencies that make up the ring is 40 hz,
and so the 'volume' of the mixture dips and
rises 40 times per second.

Busy Signals:

Similar to rings, there are three common signals that share the same
frequencies but differ in timing.  These are the busy, local reorder, and
toll congestion tones.  They are composed of (again, an equal amplitude
mix of) 480 and 620 hz tones.  The busy signal is on for half of a second,
and off for half of a second.  The other two are shorter in duration and
their off times are shorter, too.