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
- Busy tone or busy signal
- Call waiting tone
- Comfort tone
- Conference call tone
- Confirmation tone
- Congestion tone
- Dial tone
- Disconnect tone
- End of three-party service tone (three-way calling)
- Executive override tone
- Holding tone
- Preemption tone
- Queue tone
- Recall dial tone
- Record tone
- Recorder warning tone
- Reorder tone
- Audible ringing (ringing tone)
- Power ringing
- Second dial tone
- Special dial tone
- Special information tone (SIT)
- Stutter dial tone (used as a message-waiting indicator)
- Waiting tone
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 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 |