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A vertical service code (VSC) is a sequence of digits and the signals star (*) and pound/hash (#) dialed on a telephone keypad or rotary dial to access certain telephone service features. [1]
Hanging up the phone after this would result in a ring back. When the ring back was answered, the ring back process could be repeated by hanging up and lifting up the phone quickly again, and then hanging up. Most other numbers listed for ringback are specific to one exchange or one telco; available lists tend to be outdated and unreliable.
For example, calling a US phone in Europe may return a European ringback tone or vice versa. Increasingly, networks may opt to play their own domestic tones instead, making roaming seamless. In this case the ringing state is sent by the host network and the tone is generated by the home network.
A ring generator or ringing voltage generator is a device which outputs 20 cycle sinusoidal AC at up to 110 volts peak to power bells or annunciators in one or more telephone extensions. [4] The output stops if a handset is taken off the hook. In terminology devised by phone phreaks, a ringing generator is a magenta box.
A telephone keypad is a keypad installed on a push-button telephone or similar telecommunication device for dialing a telephone number. It was standardized when the dual-tone multi-frequency signaling (DTMF) system was developed in the Bell System in the United States in the 1960s – this replaced rotary dialing , that had been developed for ...
In many cases, the cadence consists of a double ring of 0.4 seconds separated by 0.2 seconds, and a two second pause after which the cadence repeats (0.4s on, 0.2 s off, 0.4 s on, 2 s off). In many cases the tone is a combination of 400 Hz and 450 Hz sine waves.
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 911, 112 or 999), in compliance with the law in most places.
The ringing pattern is known as ring cadence, in which the high voltage ring current is switched on and off to create the pattern. In North America, the standard ring cadence is two seconds of ringing followed by four seconds of silence. In Australia and the UK, the standard ring cadence is 400 ms on, 200 ms off, 400 ms on, 2000 ms off.