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A dial tone is a 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 procedure is invoked, often eliciting a special information tone and an Intercept message.
Listen to a dial tone from North America.
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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 played to the caller when the telephone set was placed off-hook, indicating that the system was live and a telephone number could be dialed. 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 call or talk button.
Invented by the engineer August Kruckow, the dial tone was first used in 1908 in Hildesheim, Germany. In the United States, the dial tone was introduced in the 1940s, and became widespread in the 1950s. When President Dwight D. Eisenhower retired in 1961 it was nearly universal, but the president himself had not been confronted with a dial tone. When he picked up his own household phone his assistant had to explain what the strange noise was, as well as how to use a rotary dial phone.
Before modern electronic telephone switching systems came into use, dial tones were usually generated by electromechanical means; 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 dialling 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, being a "buzz" of two interfering tones (350 Hz and 440 Hz, as defined in the Precise Tone Plan) in the North American Numbering Plan (most of North America), and a constant single tone (425 Hz) in most of Europe. Modern UK dialling tone is also 350 plus 440 Hz. Modems, fax machines, and auto dialers must be designed to recognise these so-called call-progress tones, as well as comply with differing standards and regulations.
Digital cellular telephone services, such as the GSM system, do not generate dial tones.
Private or internal PBX or key phone systems also have their own dial tone, sometimes the same as the external PSTN one, and sometimes different so as to remind users to dial a prefix for—or select in another way—an outside telephone line. ROLM CBX telephone systems were the most unique with its special internal dialtone consisting of 525 Hz and 660 Hz; which is Bell standard dial tone both multiplied by 1.5 each. ROLM also used a special busy signal consisting of 770 Hz and 880 Hz for phone in the DoNotDisturb or DND mode. This special busy would allow an attendant (the ROLM ATC console or operator OR a phone with a special Class of Service, or COS, to know they could use a special dial code in the COS to override such DND and make the phone ring.
A secondary dial tone, or second dial tone, is a dial tone-like sound presented to the caller after a call has already been set up. Secondary dial tones are often used in call queuing and call forwarding systems.
Unlike a normal dial tone, a secondary dial tone is provided when a connection has already been established and, except for free calls, is being charged for. Systems using secondary dialtone have been criticised for misleading callers into thinking that they are not yet being charged.
A "soft" dial tone, less often called "secondary" dial tone or "express" dial tone, is audibly the same as a regular one, except that there is no actual service active on the line, and normal calls cannot be made. It is maintained only so that an attached phone can dial the emergency telephone number (such as 911, 112 or 999), in compliance with the law in most places. It can sometimes 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.
Often, a new telephone number is assigned to the line so that it can function, but callback is restricted, and end-users do not know the number. These numbers may be outside the normal range used for regular lines, potentially causing trouble when telephone numbering plans are changed.
Deactivated telephone circuits can also be maintained with no dial tone at all, while still connected to and powered by the switch, in a state sometimes called INB or Installation Busy.