CallKit - ios 10

//Header File

<CallKit/CXCallObserver.h>

CXCallObserver *callObserver = [[CXCallObserver alloc] init];

// If queue is nil, then callbacks will be performed on main queue

[callObserver setDelegate:self queue:nil];

// Don't forget to store reference to callObserver, to prevent it from being released

self.callObserver = callObserver;

// get call status
- (void)callObserver:(CXCallObserver *)callObserver callChanged:(CXCall *)call {
    if (call.hasConnected) {
        // perform necessary actions
    }
}