First of all, open the Settings app on your iPhone and select Apps at the bottom of the screen. Then scroll down and tap Phone. Next, tap Incoming Calls. It should say Banner next to it ...

When your iPhone is unlocked, most of the notifications you'll receive come in the form of banners - messages that appear at the top of your screen. Banner notifications are used for all kinds of ...

It should say Banner next to it, as this is how iPhone currently displays incoming call alerts at the top of the screen. Now, to make sure an incoming call alert fills the screen, select Full Screen.