Telephone line signalling

Dial tone

A tone advising that the exchange is ready to receive call information and inviting the user to start sending call information.

In most countries, the dial tone is continuous. In a few countries, it is a slow period tone (where the On period is shorter than 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, continuous.

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