Call-progress tone

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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.

Contents

Telecommunication equipment such as fax machines and modems are designed to recognize certain tones, such as dial tone and busy tone.

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. Many European systems follow the recommendations of the European Telecommunications Standards Institute (ETSI).

Types

Regional and national standards

National telephone systems define tones to indicate the status of lines, equipment, or the result of calls with special tones. Such tones are generally standardized in each country, and may consist of single or multiple frequencies.

Most European countries have harmonised to a system of tones based on a single 425 Hz, while the United States uses a dual-frequency system. Exceptions exist, notably in private networks and on some voice over IP (VoIP) equipment. In Europe, some countries maintain national exceptions based on established older standards and have not fully adopted ETSI recommendations.

The most notable exceptions are found in the U.K. (distinct system of tones see table below), Ireland (British style ringback tone), Italy (non continuous dial tone) and France (tone frequencies at 440Hz).

Outside of Europe and North America, tone plans vary from country to country but are usually similar enough to be easily recognised by end users calling from abroad and by automatic dialling equipment, such as fax machines and modems.

Many countries have adopted plans similar to those recommended by the ETSI, others have influences from North American or British standards, while some like Japan and Australia are unique to those countries.

In many cases, when calling from abroad, busy, reorder and other call failure tones may be played by the local switch. Modern signalling protocols like SS7 send this information digitally; thus only a ringback tone or announcement generated by a distant switch in a foreign network will ever be heard by callers from other countries or networks.

Mobile phones roaming on a foreign network will often be provided with a ringback tone from the network they are temporarily hosted on. For example, calling a US phone in Europe may return a European ringback tone or vice versa. Increasingly, networks may opt to play their own domestic tones instead, making roaming seamless. In this case the ringing state is sent by the host network and the tone is generated by the home network.

In some instances, the tones are entirely generated by the local network or even by the telephone itself; this is increasingly common on VoIP-based services. In this case no distantly generated tones will be heard.

The use of signalling protocols rather than audible tones means that a voice channel to the distant switch is unnecessary unless a call is connected. This saves network bandwidth, switch capacity and is often more user friendly as it can provide local tones or even on-screen feedback to end users.

North American tones

EventLow frequencyHigh frequency
Busy signal (US)480 Hz620 Hz
Dial tone (US)350 Hz440 Hz
Ringing tone (US)440 Hz480 Hz
Zip tone (US)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]

ETSI guidelines (EU)

EventFq 1Fq 2Fq 3CadenceETSI Harmonised
Dial tone 425 Hz--------ContinuousYes
Special / stutter dial tone425 Hz--------On 0.5s, Off 0.05sYes
Ringing tone 425 Hz--------On 1.0s, Off 4sYes
Busy signal 425 Hz--------On 0.5s, Off 0.5sYes
Congestion or reorder tone 425 Hz--------On 0.25, Off 0.25Yes
Special information tone (SIT) 950 Hz1400 Hz1800 HzFq 1 On 0.33s, Off 0.33s, Fq 2 On 0.33s, Off 0.33s, Fq 3 On 0.33s, Off 1.0sYes
Zip tone (call waiting)425 Hz--------On 0.2, Off 0.2, On 0.2, Off 9.0Yes
Call dropped (GSM/3GPP)

Radio path not available

425 Hz--------On 0.20s, Off 0.20s repeated 3 timesYes
Release tone425 Hz--------On 0.25s, Off 0.25s (not repeated)Yes

EU national exceptions to harmonised ETSI [2]

CountryExceptionFq 1Fq 2Fq 3CadenceETSI harmonised
Ireland Ringing tone UK tone400 Hz450 Hz----Fq 1 & Fq 2 On 0.4s, off 0.2, On 0.4s, Off 2.0sNo and no plans to harmonise announced.
FranceAll tones – frequency440 Hz------------No, but harmonisation announced.
Italy Dial tone – cadence425 Hz--------On 0.2s, Off 0.2s, On 0.6s, Off 1sNo and no plans to harmonise announced.

UK tones

EventFq 1Fq 2Fq 3Cadence
Dial tone 350 Hz450 Hz----Continuous
Busy signal 400 Hz--------On 0.75s, Off 0.75s
Ringing tone 400 Hz450 Hz----On 0.4s, off 0.2, On 0.4s, Off 2.0s
Special information tone (SIT) 950 Hz1400 Hz1800 HzFq 1 On 0.33s, Off 0.33s, Fq 2 On 0.33s, Off 0.33s, Fq 3 On 0.33s, Off 1.0s

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
Dial tone E413 Hz + 438 Hzcontinuous
Ringback tone A425 Hz, amplitude modulated by 25 Hz0.4 s on, 0.2 s off, 0.4 s on, 2 s off
Ringback tone B400 Hz + 450 Hz0.4 s on, 0.2 s off, 0.4 s on, 2 s off
Ringback tone C400 Hz + 425 Hz + 450 Hz0.4 s on, 0.2 s off, 0.4 s on, 2 s off
Ringback tone D413 Hz + 438 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

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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.
  2. "Access and Terminals (AT); Public Switched Telephone Network; Support of legacy terminals by BroadBand IP equipment; Listing of the most relevant features and functionalities; Part 2: Analogue PSTN terminals" (PDF). ETSI Technical Report. ETSI TR 101 973-2 V1.1.1 (2002–12): 10 to 31. Retrieved 18 April 2021.