A dial tone is a

telephony Telephony ( ) is the field of technology involving the development, application, and deployment of telecommunication services for the purpose of electronic transmission of voice, fax, or data, between distant parties. The history of telephony is i ...

signal sent by a

telephone exchange A telephone exchange, telephone switch, or central office is a telecommunications system used in the public switched telephone network (PSTN) or in large enterprises. It interconnects telephone subscriber lines or virtual circuits of digital syst ...

private branch exchange A business telephone system is a multiline telephone system typically used in business environments, encompassing systems ranging in technology from the key telephone system (KTS) to the private branch exchange (PBX). A business telephone syst ...

(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 is a connection over a telephone network A telephone network is a telecommunications network that connects telephones, which allows telephone calls between two or more parties, as well as newer features such as fax and interne ...

. 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 In telephony, a special information tone (SIT) is an in-band international standard call progress tone consisting of three rising tones indicating a call has failed. It usually precedes a recorded announcement describing the problem.BOC Notes on th ...

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 Amer ...

, requiring the caller to hang up and redial.

History

Early telephone exchanges signaled the

switchboard operator In the early days of telephony, companies used manual telephone switchboards, and switchboard operators connected calls by inserting a pair of phone plugs into the appropriate jacks. They were gradually phased out and replaced by automated system ...

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 (; nds, Hilmessen, Hilmssen; la, Hildesia) is a city in Lower Saxony, Germany with 101,693 inhabitants. It is in the district of Hildesheim, about southeast of Hanover on the banks of the Innerste River, a small tributary of the Lei ...

, Germany. The Bell Telephone Manufacturing Company (BTMC) in Antwerp, Belgium,

Western Electric The Western Electric Company was an American electrical engineering and manufacturing company officially founded in 1869. A wholly owned subsidiary of American Telephone & Telegraph for most of its lifespan, it served as the primary equipment ma ...

'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 Dwight David "Ike" Eisenhower (born David Dwight Eisenhower; ; October 14, 1890 – March 28, 1969) was an American military officer and statesman who served as the 34th president of the United States from 1953 to 1961. During World War II, ...

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 A rotary dial is a component of a telephone or a telephone switchboard that implements a signaling technology in telecommunications known as pulse dialing. It is used when initiating a telephone call to transmit the destination telephone number ...

phone. Before modern electronic telephone switching systems came into use, dial tones were usually generated by electromechanical devices such as motor-generators or

vibrators Vibrator may refer to: Technology * Vibrator (sex toy), a device for massage or sexual pleasure used by both men and women * Vibrator (mechanical), a class of devices which create mechanical vibrations for uses such as signaling annunciators, d ...

. 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 a 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 and has the international callin ...

of the US,

Canada Canada is a country in North America. Its ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, covering over , making it the world's second-largest country by tot ...

, and various

Caribbean The Caribbean (, ) ( es, El Caribe; french: la Caraïbe; ht, Karayib; nl, De Caraïben) is a region of the Americas that consists of the Caribbean Sea, its islands (some surrounded by the Caribbean Sea and some bordering both the Caribbean Se ...

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 extremely similar, but combines 350 Hz and 450 Hz tones instead, creating a 100 Hz beat frequency. Most of

Europe Europe is a large peninsula conventionally considered a continent in its own right because of its great physical size and the weight of its history and traditions. Europe is also considered a Continent#Subcontinents, subcontinent of Eurasia ...

, as well as much of

Latin America Latin America or * french: Amérique Latine, link=no * ht, Amerik Latin, link=no * pt, América Latina, link=no, name=a, sometimes referred to as LatAm is a large cultural region in the Americas where Romance languages — languages derived f ...

and

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

, 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 A mobile phone, cellular phone, cell phone, cellphone, handphone, hand phone or pocket phone, sometimes shortened to simply mobile, cell, or just phone, is a portable telephone that can make and receive telephone call, calls over a radio freq ...

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 multiline telephone system typically used in business environments, encompassing systems ranging in technology from the key telephone system (KTS) to the private branch exchange (PBX). A business telephone syst ...

s (PBXs) or key telephone systems 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) provides Communications infrastructure, infrastructure and services for public Telecommunications, telecommunication. The PSTN is the aggregate of the world's circuit-switched telephone networks that ...

(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 In telephony, a message-waiting indicator (MWI) is a Telcordia Technologies (formerly Bellcore) term for an FSK-based telephone calling feature that illuminates an LED on selected telephones to notify a telephone user of waiting voicemail message ...

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 Most public switched telephone networks have a single emergency telephone number (sometimes known as the universal emergency telephone number or the emergency services number) that allows a caller to contact local emergency services for assista ...

(such as

911 911 or 9/11 may refer to: Dates * AD 911 * 911 BC * September 11 ** 9/11, the September 11 attacks of 2001 ** 11 de Septiembre, Chilean coup d'état in 1973 that outed the democratically elected Salvador Allende * November 9 Numbers * 911 ...

112 112 may refer to: *112 (number), the natural number following 111 and preceding 113 *112 (band), an American R&B quartet from Atlanta, Georgia **112 (album), ''112'' (album), album from the band of the same name *112 (emergency telephone number), t ...

999 999 or triple nine most often refers to: * 999 (emergency telephone number), a telephone number for the emergency services in several countries * 999 (number), an integer * AD 999, a year * 999 BC, a year Books * ''999'' (anthology) 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 For customers of some telephone companies in Canada and in the U.S., 6-1-1 is the abbreviated dialing telephone number used to report a problem with telephone service, or with a payphone. It is an N11 code of the North American Numbering Plan t ...

. 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