International call progress tones for IP-PBX and VoIP telephony
Telephone Line Signalling
Re-order tone
Also known as the Fast busy tone and the Re-order 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).
