Telephone line signalling

Congestion tone

A tone advising the caller that the groups of lines or switching equipment necessary for the setting-up of the required call or for the use of a specific service are temporarily engaged.

Typically, the congestion tone is a quick period tone, where the On period is equal to the Off period.

The tone consists of either a single tone or a combination of up to three tones. When a single tone is used, it may be modulated by a lower frequency tone.

The most prevalent, by country, is 425 Hz, 0.25/0.25 (on/off in seconds).

Also known as the Fast busy tone and the Re-order tone.

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