SIM900 GSM Module How to identify receiving person answered the call

I am doing a fall detection system for my university final year project.If fall occured My Sensor device send a data to Arduino UNO through BLE.

Arduino UNO will call a number using SIM900. If opposite person answered the call Arduino UNO has to play a recorded wave file to SIM900 line in instantaneously.

My problem is i cant identify that receiving person answered the call There is no serial output coming when a person answered call.

How can I identify that opposite party has answered the call?? (I mean press green button on cellphone/ or raised the receiver in landphone)