Telephone answer supervision circuit

Hello everyone, I need some support regarding telephone answer supervision. What circuit can be implemented in order to know for sure that the far end in a call have answered. I understand I can use a Call Progress device in order to have a ring back signal fed to my circuit but is it enough to be sure that the far end is talking once the ring back tone is ended by the CO?
I am bulding a circuit using an 8051 Ucontroller for use as a meter reading for time consume while talking on the phome.
Thanks for your ideas