My phone has an irritating habit built into the firmware, confirmed it's not fixable by a Samsung repair center and it's intentional by Samsung. During speaker phone calls the screen goes black in a minute. I'm often on 30+ minute calls and I need the screen lit up. Tried many apps in the Google Store. None worked. Finally found free "Screen On Call" by VORLAN Group in the store. It works perfectly, keeps the screen on during speaker calls as long as you want. I've used it on many calls and it never fails to work. Highly recommended if you have this issue.
I'm so happy to find something for this problem that drives me crazy...screen going dark at the worst moments. To me smartphones are more about being a computer than a phone...but you have to have one.