International call progress tones for IP-PBX and VoIP telephony
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.
