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

Common tonesEdit

Regional and National StandardsEdit

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 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 of 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. So, for example, calling a US phone in Europe may return a European ringback tone or visa 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 TonesEdit

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]

EU & Other European Countries following ETSI guidelinesEdit

Event Fq 1 Fq 2 Fq 3 Cadence ETSI Harmonised
Dial tone 425 Hz ---- ---- Continuous Yes
Special / Stutter Dial tone 425 Hz ---- ---- On 0.5s, Off 0.05s Yes
Ringing tone 425 Hz ---- ---- On 1.0s, Off 4s Yes
Busy signal 425 Hz ---- ---- On 0.5s, Off 0.5s Yes
Congestion or Reorder Tone 425 Hz ---- ---- On 0.25, Off 0.25 Yes
Special Information Tone (SIT) 950 Hz 1400 Hz 1800 Hz Fq 1 On 0.33s, Off 0.33s, Fq 2 On 0.33s, Off 0.33s, Fq 3 On 0.33s, Off 1.0s Yes
Call Waiting Tone 425 Hz ---- ---- On 0.2, Off 0.2, On 0.2, Off 9.0 Yes
Call Dropped (GSM/3GPP)

Radio path not available

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

EU National Exceptions to Harmonised ETSIEdit

Country Exception Fq 1 Fq 2 Fq 3 Cadence ETSI Harmonised
Ireland Ringing tone UK tone. 400 Hz 450 Hz ---- Fq 1 & Fq 2 On 0.4s, off 0.2, On 0.4s, Off 2.0s No & no plans to harmonise announced
France All Tones - Frequency 440 Hz ---- ---- ---- No, but harmonisation announced.
Italy Dial tone - cadence 425 Hz ---- ---- On 0.2s, Off 0.2s, On 0.6s, Off 1s No, but harmonisation in progress.

"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.

UK TonesEdit

Event Fq 1 Fq 2 Fq 3 Cadence
Dial tone 350 Hz 450 Hz ---- Continuous
Busy signal 400 Hz ---- ---- On 0.75s, Off 0.75s
Ringing tone 400 Hz 450 Hz ---- On 0.4s, off 0.2, On 0.4s, Off 2.0s
Special Information Tone (SIT) 950 Hz 1400 Hz 1800 Hz Fq 1 On 0.33s, Off 0.33s, Fq 2 On 0.33s, Off 0.33s, Fq 3 On 0.33s, Off 1.0s

Australian TonesEdit

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

Pre-answer tones Frequency definition Tone Cadence
Dial Tone A 425 Hz continuous
Dial Tone B 425 Hz, amplitude modulated by 25 Hz continuous
Dial Tone C 400 Hz + 425 Hz + 450 Hz continuous
Dial Tone D 400 Hz + 425 Hz continuous
Ring Tone A 425 Hz, amplitude modulated by 25 Hz 0.4 s on, 0.2 s off, 0.4 s on, 2 s off
Ring Tone B 400 Hz + 450 Hz 0.4 s on, 0.2 s off, 0.4 s on, 2 s off
Ring Tone C 400 Hz + 425 Hz + 450 Hz 0.4 s on, 0.2 s off, 0.4 s on, 2 s off
Busy Tone 425 Hz 0.375 s on, 0.375 s off
Number Unobtainable Tone 425 Hz 2.5 s on, 0.5 s off
Post-answer tones Frequency definition Tone Cadence
Recording 1400 Hz 0.425 s burst, 15 s interval
Call Waiting 425 Hz or 525 Hz 0.2 s on, 0.2 s off, 0.2 s on, 4.4 s off
Conference 425 Hz or 525 Hz or 1400 Hz 1.0 s on, 15 s off (1st), 0.36 s on, 15 s off (subsequent)
Number Unobtainable Tone 425 Hz 2.5 s on, 0.5 s off

ReferencesEdit

External linksEdit