By default, the screen and energy settings in Android don’t take effect during a call. The screen always dims after a few seconds and then stays blank, no matter how you configured it in the settings.
With “normal” phones this feature may be okay, but e.g. with a tablet like Samsung Galaxy Tab it can be annoying when using the loudspeaker or a headset.
Install the app Caffeine. This was the only way I found to make the display stay awake during a call.