Solution 1:

If you want to keep the Call activity still clickable, but not have any controls on your overlay, you can do this by calling

getWindow().addFlags(WindowManager.LayoutParams.FLAG_NOT_TOUCHABLE);
getWindow().addFlags(WindowManager.LayoutParams.FLAG_NOT_TOUCH_MODAL);

In the onCreate() method of the activity that is shown over the call.

The layout parameters useable together with this are:

android:windowBackground="@android:color/transparent" 
android:windowIsTranslucent="true" 
android:windowAnimationStyle="@android:style/Animation.Translucent"