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 In signal processing, a signal is a function that conveys information about a phenomenon. Any quantity that can vary over space or time can be used as a signal to share messages between observers. The '' IEEE Transactions on Signal Processing' ...

sent by a telephone exchange or private branch exchange (PBX) to a terminating device, such as a telephone, when an

off-hook In telephony, on-hook and off-hook are two states of a communication circuit. On subscriber telephones the states are produced by placing the handset onto or off the hookswitch. Placing the circuit into the off-hook state is also called ''seizing th ...

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 between the called party and the calling party. First telephone call The first telephone call was made on March 10, 1876, by Alexander Graham Bell. Bell demonstrated his ability to "ta ...

. 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 ...

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 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 L ...

, 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 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 In engineering, electromechanics combines processes and procedures drawn from electrical engineering and mechanical engineering. Electromechanics focuses on the interaction of electrical and mechanical systems as a whole and how the two systems ...

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 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 calling ...

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 nations specifies a combination of two tones (350 Hz and 440 Hz) which, when mixed, creates a

beat frequency In acoustics, a beat is an interference pattern between two sounds of slightly different frequencies, ''perceived'' as a periodic variation in volume whose rate is the difference of the two frequencies. With tuning instruments that can produce ...

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 subcontinent of Eurasia and it is located entirel ...

, 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 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 exchanges (PBXs) or

key telephone system 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 also play a dial tone to station users. It may be the same type as used by the public switched telephone network (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 A vertical service code (VSC) is a sequence of digits and the signals star (*) and number sign (#) dialed on a telephone keypad or rotary dial to enable or disable certain telephone service features. Some vertical service codes require dialing of ...

, such as

call forwarding Call forwarding, or call diversion, is a telephony feature of all telephone switching systems which redirects a telephone call to another destination, which may be, for example, a mobile or another telephone number where the desired called party ...

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 assis ...

(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 * 91 ...

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: T ...

), 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