TELECOM Digest OnLine - Sorted: Re: Invalid Number Tones

Re: Invalid Number Tones

Brad Houser (
Wed, 2 Feb 2005 11:54:24 -0800

L. Hao <> wrote in message

> I need to design an algorithm to detect call progress. For example, it
> needs to figure out whether the call hits a live person, an answering
> machine, a fax machine, and etc. But among all these things, I am
> having trouble finding the spec for this so-called invalid
> number. When you dial an invalid number, you normally get a tone
> followed by " the number you just dialed ...". Can anyone tell me where
> I can find the spec for this tone? Or better yet, if anyone can share
> his/her methods of detecting it, it'd be great.

The Special Information Tone (SIT) is 950, 1400, 1800 (all tones +- 50 hz)
for 330 +- 70 ms.

Brad Houser

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