Call-progress tone

In telephony, call progress tones are audible tones that provide an indication of the status of a telephone call to the user. The tones are generated by a central office or a private branch exchange (PBX) to the calling party.

Equipment such as fax machines and modems are designed to recognize dial tones and busy tones.

The ITU-T E.180 and E.182 recommendations define the technical characteristics and intended usage of some of these tones. ToneScript is a tone description format that may be used to specify the tone.

Common tones

National tone plans

National telephone systems define tones to indicate the status of lines, equipment, or the result of calls with special tones. Such tones are standardized in each country and may consist of single or multiple frequencies. Most European countries use a single frequency of between 400 and 450 Hz, while the United States uses a dual frequency system.

North American Tones
Event Low frequency High frequency
Busy signal (US) 480 Hz 620 Hz
Ringing tone (US) 440 Hz 480 Hz
Dial tone (US) 350 Hz 440 Hz

The tone frequencies, as defined by the Precise Tone Plan, are selected such that harmonics and intermodulation products will not cause an unreliable signal. No frequency is a multiple of another, the difference between any two frequencies does not equal any of the frequencies, and the sum of any two frequencies does not equal any of the frequencies. The frequencies were initially designed with a ratio of 21/19, which is slightly less than a whole tone. The frequencies may not vary more than ±1.8% from their nominal frequency, or the switching center will ignore the signal. The high frequencies may be the same volume as – or louder than – the low frequencies when sent across the line. The loudness difference between the high and low frequencies can be as large as 3 decibels (dB) and is referred to as "twist." The duration of the tone should be at least 537 ms.[1]

European tones
Event Low frequency High frequency
Busy signal (UK) 400 Hz ----
Busy signal (Most of Europe) 425 Hz ----
Ringing tone (UK & Ireland) 400 Hz 450 Hz
Ringing tone (Most of Europe) 425 Hz ----
Dial tone (UK) 350 Hz 450 Hz
Dial tone (Most of Europe) 425 Hz ----

Australian Tones

Supervisory tones in the Australian PSTN are defined in AS/CA S002, published by the Communications Alliance.

Pre-answer tonesFrequency definitionTone Cadence
Dial Tone A425 Hzcontinuous
Dial Tone B425 Hz, amplitude modulated by 25 Hzcontinuous
Dial Tone C400 Hz + 425 Hz + 450 Hzcontinuous
Dial Tone D400 Hz + 425 Hzcontinuous
Ring Tone A425 Hz, amplitude modulated by 25 Hz0.4 s on, 0.2 s off, 0.4 s on, 2 s off
Ring Tone B400 Hz + 450 Hz0.4 s on, 0.2 s off, 0.4 s on, 2 s off
Ring Tone C400 Hz + 425 Hz + 450 Hz0.4 s on, 0.2 s off, 0.4 s on, 2 s off
Busy Tone425 Hz0.375 s on, 0.375 s off
Number Unobtainable Tone425 Hz2.5 s on, 0.5 s off
Post-answer tonesFrequency definitionTone Cadence
Recording1400 Hz0.425 s burst, 15 s interval
Call Waiting425 Hz or 525 Hz0.2 s on, 0.2 s off, 0.2 s on, 4.4 s off
Conference425 Hz or 525 Hz or 1400 Hz1.0 s on, 15 s off (1st), 0.36 s on, 15 s off (subsequent)
Number Unobtainable Tone425 Hz2.5 s on, 0.5 s off

References
  1. Transmitted signaling rate of 93 ms per digit with minimum 40 ms pause between tones. A 40 ms tone should be detected and a 23 ms tone must be rejected.
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