Telephone line signalling

Busy tone

A tone advising the caller that the telephone number is busy.

Typically, the busy 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 at 0.5/0.5 (on/off in seconds).

Also known as the Engaged tone.

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