Editing Busy signal (section)

==Styles==
Countries have different signaling tones that act as busy signals, in most cases consisting of a tone with equal on/off periods at a rate of between 60 and 120 interruptions per minute (i.p.m.).

In North America, the [[Precise Tone Plan]] used today employs two tones of 480 and 620 [[Hertz|Hz]] at an amplitude of -24 dBm with a 0.5&nbsp;[[second|s]] on/off cadence. Prior to the adoption of the PreciseTone system, the busy signal generally had the same frequency as the [[dial tone]]. Until frequencies began to be standardized in the 1960s, telephone signals varied from telephone exchange to exchange.

The [[ETSI|European Telecommunications Standards Institute]] (ETSI) recommendation<!-- meaning 'ETSI guide', as opposed to an ETSI standard --> for busy tone is a 425&nbsp;Hz tone at -20[[dBm]] in a 0.5&nbsp;s on/off cadence. This sequence was already in use in Portugal, Spain, The Netherlands, (West-)Germany, Italy, Greece, Switzerland, Lichtenstein, Iceland, Norway, Andorra, Monaco, San Marino, and Vatican City prior to publication of the ETSI recommendation.

The ETSI recommendation is now followed by all countries of the European Union. Historical oddities within the EU are:

* [[Republic of Ireland|Ireland]] follows the ETSI recommendation for the busy tone, but the ringback tone is the same as that used in the UK.
* A few networks in France continue to use a 440&nbsp;Hz/0.5&nbsp;s sequence in place of the ETSI recommended 425&nbsp;Hz/0.5&nbsp;s sequence, but this is gradually being replaced.
The ETSI recommendation is also the default (i.e. non-localized) busy tone generated by mobile phones that follow the [[3GPP|GSM & 3GPP family of standards]].

The ETSI recommendation is also followed by some [[ISDN]] equipment and [[Business telephone system|PBX/office systems]] found outside Europe.

Most countries of the former Soviet Union, including Russia, are not members of the ETSI. These former Soviet republics employ a 425&nbsp;Hz busy tone with a 0.4&nbsp;s on/off cadence.

In the United Kingdom, the busy tone is a single 400&nbsp;Hz tone with equal 0.375&nbsp;s on/off periods. This was the case even when the UK was still part of the EU. The current 400&nbsp;Hz/375ms tone was adopted in the mid-to-late 1960s and replaced the older busy tone, which was the same 400&nbsp;Hz signal but at half the pulse duration, 0.75&nbsp;s on, 0.75&nbsp;s off.