Updates are sent out by Google to the phone's manufacturer, in this case LG. Then they are sent up the ladder to your phone's branded service provider, again in this case AT&T. Lg will add their changes to the OS and so will AT&T when they get it. In the case of you having service with Straight Talk, it doesn't matter because ST still uses AT&T towers and service. So when AT&T pushes out the update for this phone, you will get it also because technically speaking, you are still on AT&T's network.
For the two different screens, Blitz is correct. There are two call screens. One when the phone is locked and one when its not (home screen). Hope that explains it for you.