找出呼叫何时开始振铃

I have an android app from which I can start calls. Id like to know if it is possible to know when does the actual ringing start. Sth like TelephonyManager.CALL_STATE_OFFHOOK is not enough, as its triggered already as i start dialing, few secs before the ringing I want starts. ThanksI have an android app from which I can start ca