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.

Types

Major ones encountered by the public on a regular basis are listed in bold:

• 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.

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:

References

External links

• Various Tones Used in National Networks (According to ITU-T Recommendation E.180)