Telephone System Status Tones:

The following are the tones used in the North American phone system for the
essential status indicator messages. All of them are dual-tone (that is, they
actually combine two frequency tones together). Note that these are indeed
the North American standards, and they do vary across the planet.

Dial tone: The dial tone is 350 Hz plus 440 Hz, played constantly without
interruption. It indicates that the CO (Central Office) is ready to accept
dialing digits from the user.

Busy signal: The busy signal is 480 Hz plus 620 Hz, played at a rate of 60
IPM (Interruptions Per Minute), being on for 0.5 seconds and off for 0.5
seconds. It indicates that the called number exists, but is currently in use.

Reorder signal: The reorder signal uses the exact same frequencies as the
busy signal (480 Hz plus 620 Hz), but is twice as fast at 120 IPM
(Interruptions Per Minute), being on for 0.25 seconds and off for 0.25
seconds. (For this reason, the reorder signal is sometimes called a "fast
busy" signal.) A signal not well understood by the general public, the
reorder signal is often mistaken for a regular busy signal, but in fact, it
does not indicate that the called line is busy at all; Rather, it indicates
that the telephone switches between the caller and the destination line are
too busy to handle the call. Essentially, it means that the telephone system
is busy, not the called number. Note that this really is spelled "reorder" as
in "to change the order of", not "recorder" as in "to make a recording of
something".

Ring: The ringing sound played when a number has been dialed and the
destination line is ringing consists of 440 Hz plus 480 Hz, which is played
in a pattern of 2 seconds on, and 4 seconds off. This is sometimes called the
"audible ringback" sound.