A Ring-back Tone (RBT), or Audible Ringing Tone or Ring-back Signal, is the audible ringing that is heard on the telephone line by the calling party after dialing and prior to the call being answered at the receiving end. This tone assures the calling party that a ringing signal is being sent on the called party's line, although the ring-back tone may be out of sync with the ringing signal.

Ring-back tone characteristics

The ring-back tone is different in various countries depending on the requirements for the ring-back specification in those countries. The ring-back signal may be generated by the called-party servicing switch(S-MSC) or by the calling-party switch (O-MSC), but it is not generated by the called telephone instrument. It is generally started and stopped at the same rate as the ringing signal itself but perhaps out of phase.

United Kingdom and the Commonwealth

In the UK and many other Commonwealth countries, it is a signature double beep. 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. The pulse is made by mixing a 400 Hz and 450 Hz sine wave.

This tone has been made famous by being included at the end of Pink Floyd's "Another Brick in the Wall (Part 2)" and "Young Lust". It is also used in Spacehog's song "In the meantime", as well as The Penguin Cafe Orchestra's "Telephone and Rubber Band".

North American Numbering Plan

In the NANP (United States, Canada, and others), the standard Audible Ring or ring-back signal is generated by summing a 440 Hz tone with a 480 Hz tone and applying these to the telephone line in a 2 second on and 4 second off cadence. The tone combination produces a warbling "ring... ring..." sound, caused by the 40Hz beat between the two.

Rest of the world

Most other countries use a single tone, as do some PBX and key phone systems.

In some countries, for example India, ringback tone is referred to as dialer tone.

Personalized ring-back tone

In recent years "personalized" ring-back tones have become globally popularFact: date=September 2008. With this feature, callers will hear an audio selection applied to the telephone line that has been previously determined by the called party. Audio selections can include music, messages, greetings and special effects like mixing. Equipment is installed in the telephone network to enable replacement of the standard ring-back tone with a personalized audio selection. This ensures that the handset equipment is not burdened with media storage and playback .The application of the personalized audio selections is accomplished with a subscriber account that can be modified through WWW, WAP, SMS, USSD or Voice User Interfaces, as well as by customer care personnel.

The first personalized ring-back tone service called Color Ring Back Tone (CRBT) was invented by a South Korean company, Witcom in May 2001 and started service with SK Telecom in South Korea from April 2002. Witcom also developed CRBT IP server for mass service as well. In March 2004, Vodafone D2 was the first European operator to launch the Ringback Tone service. Vodafone used the European Computer Telecoms (ECT) solution.