So i want a very clean methods to detect that the call has been answered or hangup

Hehe, so would every manufacturer of CTI predictive dialing equipment! Without resorting to CTI processing boards or DSP-based algorithms, I suppose you would infer an answer after ringback stops (timeout) like you mention. Depending on the frequency range and responsiveness of the call progress (or energy) detector, sometime after detecting ringback you might be able to detect faster or more irregular signal transitions and infer that you are hearing "voice".

For what it's worth, here is a call progress algorithm that uses cadence, not frequency, and is meant to be "wrapped" up inside a higher level that calls it multiple times as required to transition through call progress phases. You might be able to tweak it to return SIG_VOICE!

Get some more ideas by Googling for things like "predictive dialing" and "dialing answer detection" -- too much stuff for me to look through for you.