Dial tone
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A dial tone (known in the British Isles as a dialling tone) is a telephony signal used to indicate that the telephone exchange is working, has recognized an off-hook, and is ready to accept a call. The tone stops when the first numeral is dialed. If no digits are forthcoming, the permanent signal procedure is invoked, often eliciting a special information tone.
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[edit] History
In the 20th century when automated telephone systems were first being deployed, telephone companies noticed that customers were often confused by the apparent lack of response (before this, a telephone operator would answer), and would often assume the phone was not working. To avoid this, exchange systems would play a small buzzing sound into the line instead.
In the United States, this change began in late 1947. It became widespread during the administration of President Dwight D. Eisenhower. By the time of Eisenhower's retirement, it was nearly universal, but the president himself had little experience with real life since being in such positions of power, and upon retiring he was confronted with a dial tone the first time he picked up his household phone, and ended up needing his assistant to explain what the strange noise was, as well as how to use a rotary phone dial[1].
Before modern electronic telephone switching systems came into use, dial tones were usually generated by electromechanical means; in the United States, the standard "city" dial tone consisted of a 600 Hz tone amplitude-modulated at 120 Hz. Some dial tones were simply adapted from 60 Hz AC line current. In the UK, the standard Post Office dial tone was 33 Hz: this was generated by a motor-driven ringing machine in most exchanges, and by a vibrating-reed generator in the smallest.
The modern dial tone varies between countries, being a "buzz" of two interfering tones (350 Hz and 440 Hz, as defined in the Precise Tone Plan) in the NANP (most of North America), and a constant single tone (425 Hz) in most of Europe. Modems, fax machines, and autodialers must be designed to recognise these so-called call-progress tones, as well as comply with differing standards and regulations.
Private or internal PBX or key phone system also have their own dial tone, sometimes the same as the external PSTN one, and sometimes different to remind users to dial a prefix for an outside telephone line.
A "stuttered" or interrupted dial tone may mean that there is voice mail waiting (see Message Waiting Indicator), or may occur to confirm that a calling feature such as call forwarding has been activated.
Mobile phones do not have dial tones, as the user instead presses "send" to actually go off-hook and complete the call once the desired phone number is entered.
Experienced musicians will be able to tune their stringed instruments to a U.S. dial tone, since its two frequencies correspond to the standard concert pitch of A440, and (approximately) the "F" below.
Example of a North American dial tone
[edit] "Soft" dial tones
A "soft" dial tone is audibly the same as a regular one, except that there is no actual service active on the line, and normal calls cannot be made. It is maintained only so that an attached phone can dial the emergency telephone number (such as 9-1-1 or 1-1-2), in compliance with the law in most places. It can sometimes call the business office of the Local exchange carrier which owns or last leased the line, such as via 6-1-1. Other functions such as ringback or ANAC may also be accessed by technicians in order to facilitate installation or activation.
Often, a new telephone number is assigned to the line so that it can function, but callback is restricted, and end-users do not know the number. These numbers may be outside the normal range used for regular lines, potentially causing trouble when telephone numbering plans are changed.
Deactivated lines can also be maintained with no dial tone at all, while still connected to and powered by the switch, in a state sometimes called INB or Installation Busy.
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