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Adial tone (dialling tone in theUK) is atelephony signal sent by atelephone exchange orprivate branch exchange (PBX) to a terminating device, such as a telephone, when anoff-hook condition is detected. It indicates that the exchange is working and is ready to initiate atelephone call. The tone stops when the first dialed digit is recognized. If no digits are forthcoming, thepartial dial procedure is invoked, often eliciting aspecial information tone and anintercept message, followed by theoff-hook tone, requiring the caller to hang up and redial.
Early telephone exchanges signaled theswitchboard 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 inHildesheim, Germany.[1]
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 PresidentDwight 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 arotary dial phone.[2]
Before modern electronic telephone switching systems came into use, dial tones were usually generated byelectromechanical devices such asmotor-generators orvibrators. In the United States, the standard "city" dial tone was a 600Hz tone that was amplitude-modulated at 120 Hz.[3] 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. ThePrecise Tone Plan for theNorth American Numbering Plan of theUS,Canada, and variousCaribbean nations specifies a combination of twotones (350 Hz and 440 Hz) which, when mixed, creates abeat 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 ofEurope, as well as much ofLatin America andAfrica, 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.
Private branch exchanges (PBXs) orkey telephone systems also play a dial tone to station users. It may be the same type as used by thepublic 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.
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.
A stutter dial tone is a rapidly interrupted tone used to indicate special service conditions. It may serve as amessage-waiting indicator for voice mail, or indicate that acalling feature, such ascall forwarding has been activated.
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 theemergency telephone number (such as911,112 or999), in compliance with the law in most places. It can sometimes also call the business office of thelocal exchange carrier which owns or last leased the line, such as via6-1-1. Other functions such asringback orANAC may also be accessed by technicians in order to facilitate installation or activation.
