View source for Dial tone

{{Short description|Signal indicating that a call is possible}}
{{For|the G.I. Joe character|Dial Tone (G.I. Joe)}}
{{More citations needed|date=August 2007}}

A '''dial tone''' ('''dialling tone''' in the [[United Kingdom|UK]]) is a [[Signaling (telecommunications)|telephony signal]] sent by a [[telephone exchange]] or [[private branch exchange]] (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]]. The tone stops when the first dialed digit is recognized. If no digits are forthcoming, the [[permanent signal|partial dial]] procedure is invoked, often eliciting a [[special information tone]] and an [[intercept message]], followed by the [[off-hook tone]], requiring the caller to hang up and redial.

==History==
{{Listen
|filename=US dial tone.ogg
|title=Example of a North American dial tone
|description=Listen to a dial tone from North America of 350&nbsp;Hz + 440&nbsp;Hz.
|filename2=UK_dial_tone_example.ogg
|title2=Example of a UK/British dial tone
|description2=Listen to a dial tone from the United Kingdom of 350&nbsp;Hz + 450&nbsp;Hz.
|filename3=1TR110-1 Kap8.1 Waehlton.ogg
|title3=Example of a current European dial tone
|description3=Listen to a European dial tone of 425&nbsp;Hz
|filename4 = Sine wave 440.ogg
|title4   = Example of a French dial tone
|description4=Listen to a French dial tone of 440&nbsp;Hz
|filename5=JA dial tone.ogg
|title5=Example of a Japanese dial tone
|description5=Listen to a Japanese dial tone of 400&nbsp;Hz
}}

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]], Germany.<ref>{{cite news|last=Engber|first=Daniel|title=Who Made That Dial Tone?|url=https://www.nytimes.com/2014/01/12/magazine/who-made-that-dial-tone.html|access-date=10 January 2014|newspaper=[[The New York Times]]|date=12 January 2014}}</ref>

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.<ref>{{cite web|url=http://www.nps.gov/history/museum/exhibits/eise/PopCulture/gadgetsGizmos/EISE2728_grn_3421_red_9197_.html |title=Eisenhower National Historic Site |publisher=Nps.gov – U.S. National Park Service |access-date=2010-09-06 |url-status=dead |archive-url=https://web.archive.org/web/20090923011742/http://www.nps.gov/history/museum/exhibits/eise/PopCulture/gadgetsGizmos/EISE2728_grn_3421_red_9197_.html |archive-date=September 23, 2009 }}</ref>

Before modern electronic telephone switching systems came into use, dial tones were usually generated by [[electromechanical]] devices such as [[motor–generator]]s or [[vibrator (electronic)|vibrators]]. In the United States, the standard "city" dial tone was a 600&nbsp;[[hertz|Hz]] tone that was amplitude-modulated at 120&nbsp;Hz.<ref name="ElmerCat">{{cite web |title=Dial Tone – Elmer Cat |url=http://elmercat.org/phone/dialtone/ |access-date=11 January 2021}}</ref> Some dial tones were simply adapted from 60&nbsp;Hz AC line current. In the UK, the standard Post Office dialing tone was 33&nbsp;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&nbsp;Hz dial tone.

The modern dial tone varies between countries. The [[Precise Tone Plan]] for the [[North American Numbering Plan]] of the [[United States|US]], [[Canada]], and various [[Caribbean]] nations specifies a combination of two [[Pitch (music)|tone]]s (350&nbsp;Hz and 440&nbsp;Hz) which, when mixed, creates a [[Beat (acoustics)|beat frequency]] of 90&nbsp;Hz. The UK dial tone is similar, but combines 350&nbsp;Hz and 450&nbsp;Hz tones instead, creating a 100&nbsp;Hz beat frequency. Most of [[Europe]], as well as much of [[Latin America]] and [[Africa]], uses a constant single tone of 425&nbsp;Hz. France currently uses a single 440&nbsp;Hz tone and Japan uses a single 400&nbsp;Hz tone.

[[Cellular telephone]] services do not generate dial tones as no connection is made until the entire number has been specified and transmitted.{{Citation needed|date=July 2025}}

Although based on different technology, many [[VoIP phone]]s provide a dial tone when the handset is lifted or speakerphone is activated.  The dial tone doesn't originate from a central exchange, but is played from a recording stored in the phone's software.   It is provided to indicate, especially to users familiar with older phone technology, that the phone is functioning and ready for the user to place a call.<ref>{{cite web|title=No Dial Tone Troubleshooting|url=https://www.whichvoip.com/articles/no-dial-tone-troubleshooting.htm |access-date=9 July 2025}}</ref>

==Variants==
===Second dial tone===
In many countries a second dial tone was generated if the user dialled an area code (as opposed to a local number), indicating that the remote exchange was ready to receive digits. This was not necessary where the originating exchange had a register large enough to store both an area code and a subscriber number. The electronic exchanges of the later 20th century all had such registers, but since there were often still some electromechanical exchanges in rural areas, the second dial tone was generally only phased out in the 1990s or early 2000s when all exchanges became digital. The second dial tone would usually be different, for example somewhat higher, than the initial dial tone.

[[Private branch exchange]]s (PBXs) or [[key telephone system]]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 an outside line by some other method.

===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]] (such as [[9-1-1|911]], [[112 (emergency telephone number)|112]] or [[999 (emergency telephone number)|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 number|ringback]] or [[Automatic number announcement circuit|ANAC]] may also be accessed by technicians in order to facilitate installation or activation.

==See also==
{{portal|Telephones}}
* [[Ringback tone]]
* [[Busy signal]]

==References==
{{Commons category|Dial tones}}
{{Reflist}}

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{{Telecommunications}}

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[[Category:Telephony signals]]

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