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A dial tone (known in the British Isles as a dialling tone) is a telephony signal used to indicate that the telephone exchange is working, has recognized an off-hook, and is ready to accept a call. The tone stops when the first numeral is dialed. If no digits are forthcoming, the permanent signal procedure is invoked, often eliciting a special information tone.
In the early days of telephony a telephone operator would answer when a subscriber picked up the telephone to make a call. When operators were replaced by automated systems an automatically generated tone was substituted to signify that the system was live and a call could be dialed. It was required to pick up the receiver (go off-hook) before dialing, and each digit was transmitted as it was dialed. This is unlike modern telephones, which also allow entering a number without the need to first go off-hook. A modern telephone stores the digits as they are entered, and only goes off-hook and actually sends the digits when the subscriber presses a "Call" or "Talk" button.
In the United States, 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 consisted of a 600 Hz tone 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 generated dialling tone at 50 Hz.
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.
Dial tones are not relevant to GSM cellular phones, and GSM subscribers will never hear a dial-tone. Subscribers place a call much like they would with a modern telephone, by dialing the digits and pressing a "Call" or "Talk" button. If the mobile subscriber is connected to a mobile network, then the call will be placed.
Listen to a dial tone from North America.
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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 to remind users to dial a prefix for—or select in another way—an outside telephone line.
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 criticized 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 9-1-1, 1-1-2 or 9-9-9), 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 lines 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.