Dial tone
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A dial tone is a telephony signal used to indicate that the telephone exchange is working and ready to accept a call. When automated telephone systems were first being deployed they noticed that customers were often confused by the apparent lack of response (before this, an operator would answer) and would often feel the phone was not working. To avoid this, exchange systems would play a small buzzing sound into the line instead.
The dial tone varies between countries, being a "buzz" of two interfering tones (350Hz and 440Hz) forming a concert F in the NANP (most of North America), and a constant single tone (425Hz) in most of Europe. Modems must be designed to recognise these, as well as comply with differing standards and regulations. 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 600Hz tone modulated at 120Hz. Some dial tones were simply adapted from 60Hz AC line current.
Private or internal PBX systems also have their own dial tone, sometimes the same as the external PSTN one, and sometimes different to remind users to dial a 9 for an outside line.
A "stuttered" dial tone may mean that there is voice mail waiting, or may occur to confirm that a 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.
"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 8-1-1. 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.
See also: dial, 2600 The Hacker Quarterly