Start of the call indication to find call duration

Hi,

I need to find out call duration of a call made and feed it to an external device.

For this I started using Gammu/Gnokii but with these I can only find out that phone is busy or not.

I can not findout at what point the other end has picked up the call and actual conversation started.

I don't mind some hardware hack.

Please can any one tell me how do I know that other end has picked up the call so that I can start the counter and calculate phone duration.