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Dial tone is a "signal" to the calling subscriber that he can dial the called subscribers number and that circuits are available at the exchange to make the call!
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 and ready to accept a call. The tone stops when the first numeral is dialed, or if there is no response after going off-hook (a timeout), when it is usually followed by a special information tone.
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. 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.
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 line.
A "stuttered" dial tone may mean that there is voice mail waiting, 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 complete the call once the desired phone number is entered.
An experienced guitar player will be able to tune to the dial tone of a phone. The dial tone is a combination of the musical notes 'A' and 'F'.
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"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 carrier company 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.
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