Anyone who needs ANSWER SUPERVISION should look at the QUINTUM A800 and A400 Analog VoIP gateways. The ANSWER SUPERVSION is PERFECT and it’s INCLUDED !!
There is a good document on quintum’s web site that should talk about this, but it is, for the most part, voice detection. It looks for different tones, ringing, busy, fast busy, intercept and voice frequency. There is a time as well since voice detection is not 100% accurate.