Call-progress tone

• Busy tone (or busy signal)
• Comfort tone
• Conference call tone
• Confirmation tone
• Congestion tone
• Dial tone
• Disconnect tone
• End of three-party service tone (three-way calling)
• Executive override tone
• Holding tone
• Preemption tone
• Queue tone
• Recall dial tone
• Record tone
• Recorder warning tone
• Reorder tone
• Ringing tone, audible ringing
• Ring tone, power ringing
• Second dial tone
• Special dial tone
• Special information tone (SIT)
• Stutter dial tone (used as a message-waiting indicator)
• Zip tone (or Call waiting tone)

National telephone systems define tones to indicate the status of lines, equipment, or the result of calls with special tones.

Such tones are generally standardized in each country, and may consist of single or multiple frequencies.

Most European countries have harmonised to a system of tones based on a single 425 Hz, while the United States uses a dual frequency system.

Exceptions exist, notably in private networks and on some VoIP equipment. In Europe, some countries maintain national exceptions based on established older standards and have not fully adopted ETSI recommendations.

The most notable exceptions are found in the U.K. (distinct system of tones see table below), Ireland (British style ringback tone), Italy (non continuous dial tone) and France (tone frequencies at 440Hz).

Outside of Europe and North America, tone plans vary from country to country but are usually similar enough to be easily recognised by end users calling from abroad and by automatic dialling equipment, such as fax machines and modems.

Many countries have adopted plans similar to those recommended by the ETSI, others have influences of North American or British standards, while some like Japan and Australia are unique to those countries.

In many cases, when calling from abroad, busy, reorder and other call failure tones may be played by the local switch. Modern signalling protocols like SS7 send this information digitally, thus only a ringback tone or announcement generated by a distant switch in a foreign network will ever be heard by callers from other countries or networks.

Mobile phones roaming on a foreign network will often be provided with a ringback tone from the network they are temporarily hosted on. So, for example, calling a US phone in Europe may return a European ringback tone or visa 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.

In some instances, the tones are entirely generated by the local network or even by the telephone itself, this is increasingly common on VoIP based services. In this case no distantly generated tones will be heard.

The use of signalling protocols rather than audible tones means that a voice channel to the distant switch is unnecessary unless a call is connected. This saves network bandwidth, switch capacity and is often more user friendly as it can provide local tones or even on-screen feedback to end users.

• Various Tones Used in National Networks (According to ITU-T Recommendation E.180)