A dial tone (dialling tone in the UK) is a telephony signal sent by a

telephone exchange A telephone exchange, telephone switch, or central office is a central component of a telecommunications system in the public switched telephone network (PSTN) or in large enterprises. It facilitates the establishment of communication circuits ...

private branch exchange A business telephone system is a telephone system typically used in business environments, encompassing the range of technology from the key telephone system (KTS) to the private branch exchange (PBX). A business telephone system differs from ...

(PBX) to a terminating device, such as a telephone, when an off-hook condition is detected. It indicates that the exchange is working and is ready to initiate a

telephone call A telephone call, phone call, voice call, or simply a call, is the effective use of a connection over a telephone network between the calling party and the called party. Telephone calls are the form of human communication that was first enabl ...

. The tone stops when the first dialed digit is recognized. If no digits are forthcoming, the partial dial procedure is invoked, often eliciting a special information tone and an

intercept message An intercept message is a telephone recording informing the caller that the call cannot be completed, for any of a number of reasons ranging from local congestion, to disconnection of the destination phone, number dial errors or network trouble a ...

, followed by the

off-hook tone The off-hook tone (also off-hook warning, howling tone, or howler tone) is a telephony signal for alerting a user that the telephone has been left off-hook without use for an extended period, effectively disabling the telephone line. North Ame ...

, requiring the caller to hang up and redial.

History

Early telephone exchanges signaled the switchboard operator when a subscriber picked up the telephone handset to make a call. The operator answered requesting the destination of the call. When manual exchanges were replaced with automated switching systems, the exchange generated a tone to the caller when the telephone set was picked up, indicating that the system was ready to accept dialed digits. Each digit was transmitted as it was dialed which caused the switching system to select the desired destination circuit. Modern electronic telephones may store the digits as they are entered, and only switch off-hook to complete the dialing when the subscriber presses a button. Invented by engineer August Kruckow, the dial tone was first used in 1908 in

Hildesheim Hildesheim (; or ; ) is a city in Lower Saxony, in north-central Germany with 101,693 inhabitants. It is in the district of Hildesheim (district), Hildesheim, about southeast of Hanover on the banks of the Innerste River, a small tributary of t ...

, Germany. The Bell Telephone Manufacturing Company (BTMC) in Antwerp, Belgium, Western Electric's international subsidiary, first introduced dial tone as a standard facility with the cutover of the 7A Rotary Automatic Machine Switching System at Darlington, England, on 10 October 1914. Dial tone was an essential feature, because the 7A Rotary system was a common control switching system. It used the dial tone to indicate to the user that the switching system was ready to accept digits. In the United States, dial tone was introduced in the 1920s. By the time President Dwight D. Eisenhower retired in 1961 it was nearly universal, but the president himself had never encountered a dial tone. When he picked up his own household phone, his assistant had to explain what the strange noise was, as well as show Eisenhower how to use a rotary dial phone. Before modern electronic telephone switching systems came into use, dial tones were usually generated by

electromechanical Electromechanics combine processes and procedures drawn from electrical engineering and mechanical engineering. Electromechanics focus on the interaction of electrical and mechanical systems as a whole and how the two systems interact with each ...

devices such as motor-generators or vibrators. In the United States, the standard "city" dial tone was a 600 Hz tone that was amplitude-modulated at 120 Hz. Some dial tones were simply adapted from 60 Hz AC line current. In the UK, the standard Post Office dialing tone was 33 Hz; it was generated by a motor-driven ringing machine in most exchanges and by a vibrating-reed generator in the smaller ones. Some later ringing machines also generated a 50 Hz dial tone. The modern dial tone varies between countries. The Precise Tone Plan for the

North American Numbering Plan The North American Numbering Plan (NANP) is an integrated telephone numbering plan for twenty-five regions in twenty countries, primarily in North America and the Caribbean. This group is historically known as World Zone 1, World Numbering Zone ...

of the US,

Canada Canada is a country in North America. Its Provinces and territories of Canada, ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, making it the world's List of coun ...

, and various

Caribbean The Caribbean ( , ; ; ; ) is a region in the middle of the Americas centered around the Caribbean Sea in the Atlantic Ocean, North Atlantic Ocean, mostly overlapping with the West Indies. Bordered by North America to the north, Central America ...

nations specifies a combination of two tones (350 Hz and 440 Hz) which, when mixed, creates a beat frequency of 90 Hz. The UK dial tone is similar, but combines 350 Hz and 450 Hz tones instead, creating a 100 Hz beat frequency. Most of

Europe Europe is a continent located entirely in the Northern Hemisphere and mostly in the Eastern Hemisphere. It is bordered by the Arctic Ocean to the north, the Atlantic Ocean to the west, the Mediterranean Sea to the south, and Asia to the east ...

, as well as much of

Latin America Latin America is the cultural region of the Americas where Romance languages are predominantly spoken, primarily Spanish language, Spanish and Portuguese language, Portuguese. Latin America is defined according to cultural identity, not geogr ...

and

Africa Africa is the world's second-largest and second-most populous continent after Asia. At about 30.3 million km2 (11.7 million square miles) including adjacent islands, it covers 20% of Earth's land area and 6% of its total surfac ...

, uses a constant single tone of 425 Hz. France currently uses a single 440 Hz tone and Japan uses a single 400 Hz tone. Cellular telephone services do not generate dial tones as no connection is made until the entire number has been specified and transmitted.

Variants

Second dial tone

Private branch exchange A business telephone system is a telephone system typically used in business environments, encompassing the range of technology from the key telephone system (KTS) to the private branch exchange (PBX). A business telephone system differs from ...

s (PBXs) or

key telephone system A business telephone system is a telephone system typically used in business environments, encompassing the range of technology from the key telephone system (KTS) to the private branch exchange (PBX). A business telephone system differs from ...

s also play a dial tone to station users. It may be the same type as used by the

public switched telephone network The public switched telephone network (PSTN) is the aggregate of the world's telephone networks that are operated by national, regional, or local telephony operators. It provides infrastructure and services for public telephony. The PSTN consists o ...

(PSTN), or it may be a different tone to remind users to dial a prefix or select by another method an outside line.

Secondary dial tone

A secondary dial tone is a dial tone-like sound presented to the caller after a call has been set up to prompt for additional digits to be dialed. Secondary dial tones are often used in call queuing and call forwarding systems.

Stutter dial tone

A stutter dial tone is a rapidly interrupted tone used to indicate special service conditions. It may serve as a message-waiting indicator for voice mail, or indicate that a calling feature, such as call forwarding has been activated.

Soft dial tones

A soft dial tone or express dial tone may be used when no actual service is active on a line, and normal calls cannot be made. It is maintained only so that an attached phone can dial the

emergency telephone number An emergency telephone number is a number that allows a caller to contact local emergency services for assistance. The emergency number differs from country to country; it is typically a three-digit number so that it can be easily remembered and ...

(such as 911, 112 or 999), in compliance with the law in most places. It can sometimes also 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.

References

{{DEFAULTSORT:Dial Tone Telephony signals ar:نغمة اتصال no:Summetone