International call progress tones for IP-PBX and VoIP telephony
Telephone Line Signalling
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- Also known as the Audible ring tone and the Ringback tone.
- A tone advising the caller that a connection has been made and that a calling signal is being applied to a telephone number or service point.
- In most countries, the ringing tone is a slow period tone (where the On period is shorter than the Off period). In a few countries (e.g. UK & ex-colonies) a distinctive double ring cadence is used.
- It 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, 1.0/4.0 (on/off in seconds).