private Call incomingCall;

private void setupIncomingCallListener() {
    spark.phone().setIncomingCallListener(new IncomingCallListener() {
        @Override
        public void onIncomingCall(final Call call) {
            incomingCall = call;
            call.setObserver(new PhoneObserver());

            // This puts the call in the ringing state.
            call.acknowledge(new CompletionHandler<Void>() {
                @Override
                public void onComplete(Result<Void> result) {
                    Log.d("Phone", "Call acknowledged");
                }
            });
        }
    });
}

public void rejectIncomingCall() {
    incomingCall.reject(new CompletionHandler<Void>() {
        @Override
        public void onComplete(Result<Void> result) {}
    });
}