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.

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 comfort noise, back then implemented as a small buzzing sound into the line.

In the United States, this changed in the 1940s. It became widespread during the administration of President Dwight D. Eisenhower. By the time of Eisenhower's retirement it was nearly universal; However, the president himself wasn't confronted with a dial tone until upon retiring he picked up his own household phone. He 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 smaller ones.

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. 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 Listen to a dial tone from North America.
Problems listening to this file? See media help.

Variants

Second dial tone

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.

Secondary dial tone

A secondary dialtone, or second dialtone, is a dialtone-like sound presented to the caller after a call has already been set up. Secondary dialtones are often used in call queuing and call forwarding systems.

Unlike a normal dialtone, a secondary dialtone is provided when a connection has already been established, and typically costing money to the call originator. Systems using secondary dialtone have been criticized for misleading callers into thinking that they are still waiting for a call to be set up.

Stutter dial tone

A "stuttered" or interrupted dial tone may be used to indicate voice mail waiting (see Message Waiting Indicator), or may occur to confirm that a calling feature such as call forwarding has been activated.

"Soft" dial tones

A "soft" dial tone, less often called "secondary" dialtone, 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.

See also

• Dual-tone multi-frequency

External links

• World PSTN Tone Database - Dial tones