Telephone Tones

A source of innocent merriment!

(W.S. Gilbert)

This page contains definitions of call progress tones and other tones used in the British telephone network.

Call Progress Tones

These are what you hear as a calling is being connected (e.g. dial tone, ringing tone).

The source of the following information is:

``On'' and ``off'' durations are in ms. The frequency is 400 Hz, except where noted.

By clicking on the [AU] ``button'', you can hear a 12-second audio sample of the tone (if you have the appropriate hardware). Technical note: The audio is not stored here; it is generated each time you request it, by a cgi (Common Gateway Interface) program. The source code of the program is here.   New - code now in C++!

    On Off On Off Notes Audio sample
BT Busy tone 375 375       [AU]
EET Equipment engaged tone 400 350 225 525 1 [AU]
RT Ringing tone 400 200 400 2000 2 [AU]
NU Number unobtainable Continuous   [AU]
PT Pay tone 125 125     3 [AU]
DT Dial tone Continuous 4 [AU]


  1. The amplitude of the 225ms tone is 6dB higher than that of the 400mS tone. This is specified (I'm reliably told) in BS 6305 (1992). I'm grateful to Nigel Roles <> for pointing this out.
  2. Frequency: 400+450 Hz.
  3. Not used any more. This really belongs in the history section!
  4. Frequency: 350+450 Hz.


If all you want is a sample of a ringing (warbling) telephone so you can annoy your colleagues, there are two, slightly different, such samples here. Try setting them off in parallel, for an authentic ``busy office'' sound!

There is no standard for ``warbles'' (so far as I know). A warble is defined by the frequencies of the two alternating tones (f1, f2), and the warble frequency (fw), specified in Hz below. In a real telephone, these values depend on the particular ringer chip used.

  f1 f2 fw Audio sample
Warble Sample 1 1500 1700 8 [AU]
Warble Sample 2 900 1100 8 [AU]

Dual Tone Multi Frequency

DTMF tones are used for ``dialling'' a number. Each digit is signalled by a 100ms burst of f1+f2 Hz, followed by 100ms silence. Some equipment will accept shorter pulse durations, or shorter silent durations, but this is not recommended.

The frequencies f1, f2 are given by the following table.

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

E.g. digit `5' is signalled by 770+1336 Hz.

Enter a string of digits from the set { 0-9, *, #, A-D } in the space below for a sample.

Here's some C++ code for generating DTMF tones, and here's some code for recognizing them.

Tony Fisher /