fixguider.com/how-to-make-caller-picture-full-screen-on-android/ explains how this can be made to happen and there are I believe some apps that can make it work too. A Google Search will bring options up.
Unfortunately there isn't a way to make it happen via the native settings in the phone I believe currently.
In Samsung One Ui you have options such as launch the Phone App, tap on the 3 dots and choose Settings and then Call Background but this will only set one Background at a time, and not really what your wanting to achieve.
If you Google Search you'll see lots of posts here, and elsewhere where people want this feature.
Perhaps send feedback via the Get Help section in your Samsung Members App.
Daily Driver > Samsung Galaxy s²⁵ Ultra 512Gb ~ Titanium Black.
The advice I offer is my own and does not represent Samsung’s position.
I'm here to help. " This is the way. "
