Phone - keeping the screen on while talking

The screen goes off as soon as you put your handset close to your ear because it's intended to prevent ending a call by your face touching the screen, or to prevent an app from accidentally starting. It turns off as the proximity sensor detects an object such as your head/ear getting close to the handset.

There is one thing you can do to end the call easily. This is done by going to settings > accessibility (that's how it's done on my Android phone). You can put a check mark on Power Button Ends Call option.