International call progress tones for IP-PBX and VoIP telephony
Telephone Line Signalling
Also known as the Intercept tone and the SIT.
A tone advising the caller that the called number cannot be reached for reasons other than "subscriber busy" or "congestion". The tone may also be used in conjunction with recorded announcements to signify that what the caller is about to hear is a recording. It should always be used to precede all call failure announcements.
In most countries, the special information tone consists of a sequence of three tones.
The most prevalent, by country, is 950/1400/1800 Hz at 3x0.33/1.0 (on/off in seconds).