app fix ASAP - detect outgoing call answered or not

We are developing a software but we need to fix a bug. Here's the problem: I want to install an app on the phone of person A. When person A calls person B, how can the app on the phone A know when person B answers the call? I need this as part of a bigger project, so just need the code or how to do that to give to my developer to continue the project. So basically I want to detect if the outgoing call is answered or not in android. The fix should work with sdk 15, and work with android version 4.0 or higher. Please do not blindly bid, write something specific about this project so I make sure your read the requirement and can do it. I will send you the app code we currently have to fix it. I will create the full milestone at the beginning, but will NOT release the payment until you send me the code to make sure you have fixed the problem.