Telephone systems provide users with feedback about
what they are doing in order to simplify operation and
reduce calling errors. This information can be in the form
of lights, displays, or ringing, but is most often some sort
of audible tone heard on the phone line. These tones are
generally referred to as call progress tones, as they indicate
what is happening to dialed phone calls. Conditions
like busy line, ringing called party, bad number, and others
each have distinctive tone frequencies and cadences
assigned them for which some standards have been
established.
Standards for call progress tones are unfortunately
applied differently in different situations or countries. The
main groups of standards could be considered to be:
(1) the United States
(2) everyone else
Information on most schemes in use is available, and
which tones will be encountered can be predicted fairly
well by reviewing a few references.
Most tone standards vary with the country of application.
In the U.S., the tones for Network are defined in Bellcore’s
“BOC Notes on the LEC Networks”, and for PBX in the
EIA’s RS-464 documentation. Outside the U.S., national
Post, Telegraph, and Telephone (PTT) organizations set
requirements for such signals, but they generally follow
similar lines as regards frequency and cadence—the best
reference in that case being the CCITT recommendations
that cover member countries (Blue Book, Volume II—
Fascicle II.2, Supplements). The example pages from the
Blue Book below show the completeness of the CCITT
documentation. The CCITT document also shows the
U.S. tone plan, but doesn’t provide the detail found in the
AT&T publication.
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