The incoming call will behave differently depending on what kind of app you are using and in what mode. Let's take youtube as an example. In portrait mode, you will get the full screen caller window and in landscape mode, you will get the notification caller window. This is not controlled by ASUS. It's standard stock android behavior.
You can reject calls with a SMS. It is in the bottom of the full screen caller window. If you get the notification caller window first, then you need to click i (in the white area) to get the full screen. I must admit that it is a bit tricky to swipe up the SMS reply some times so give it a couple of tries. This is the stock caller window so it is controlled by Google.
