Things like the proximity sensor as well as other internals are working while your making a call.
Also the heat from your face will add to this.
It's also like if a person is gaming on the phone for a while. This will make the phone heat up faster as it's working hard to provide an experience.
As long as the phone isn't warming up to an uncomfortable level it's fine.
If your concerned then a Samsung Experience Store / Samsung Service Centre can run a diagnostic test on your phone @Boogienights
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. "
