SIT is Special Information Tone
It is used supposedly to signal that a phone number is disconnected or invalid in some way. SIT tones are different from country to country and even in the USA their amplitude or timing is not standardized even though it is supposed to be.
To analyze the SIT tone in Asterisk you would need to rewrite the Dial application and build into it a signal analyzer since the SIT tone is almost always sent before an Answer signal has been sent from the carrier meaning that another application cannot be called since the Dial has not finished.
Many dialers that claim SIT tone detection do not actually do it, they just use the standard Asterisk busy detect and/or the PRI or SIP call signalling received from the carrier. Both of these methods also have their own problems as well.
With the popularity of devices like the Telezapper and people adding SIT tones to their voicemail messages, the use of SIT tone detection has become less accurate as a form of detecting invalid phone numbers.
http://en.wikipedia.org/wiki/Special_information_tone