Audible ringing (ringing tone, colloquially also ringback tone) is a signaling tone in telecommunication that is heard by the originator of a telephone call while the destination terminal is alerting the receiving party. Audible ringing is typically a repeated tone that is not necessarily synchronous with the cadence of the power ringing signal that is sent to the called party.
Audible ringing is usually generated in the switching system closest to the calling party, especially when under the control of strict implementations of Signalling System No. 7 and the Customized Application of Mobile Enhanced Logic (CAMEL) signaling system. It may also be generated in the distant switch, transmitted in-band, so that in analog networks the caller could monitor the quality of the voice path of the connection before the call is established. Remote call progress indication permits customized tones or voice announcements by a distant switch in place of the ringing tone.
The ringing tone is often also called ringback tone. However, in formal telecommunication specifications, ringback has a different definition. It is a signal used to recall either an operator or a customer at the originating end of an established telephone call. It is also needed for coin-telephone lines to ring the telephone when the customer has hung up prematurely, for example to collect required overtime deposits.
Many European countries use tones which follow the recommendation of the European Telecommunications Standards Institute. Almost all of these tones are 425 Hz; France uses 440 Hz. Typically, the pattern is 1 second of tone followed by 3 to 5 seconds of silence.
In Japan, the standard audible ringing tone is a repeating 1-second tone with a 2-second pause between. The tone has a frequency of 400 ± 20 Hz, and the amplitude modulation is 15 to 20Hz.
In North America (excluding Mexico, Central America and parts of the Caribbean), the standard audible ringing tone is a repeating 2-second tone with a 4-second pause between. The signal is composed of the frequencies 440 Hz and 480 Hz.
United Kingdom, Ireland, Commonwealth nations
In the United Kingdom, Republic of Ireland, Australia, New Zealand, and many Commonwealth nations, it is a double ring. For most countries, it consists of a 0.4-second pulse, a 0.2-second pause, a 0.4-second pulse, and a 2-second pause. In all cases except Australia, the pulse is made by mixing a 400 Hz and 450 Hz sine wave; Australia uses up to three different combinations of frequencies. The example shown is created by mixing 400, 425, and 450 Hz sine waves.
In India, the ringing tone is called caller ringback tone (CRBT), which has a duration of one minute.