@Gerry Peters: Do you have a case or cover on the device? Your Galaxy A12 has a proximity sensor which is designed to place the screen in idle mode when you hold the phone up to your ear, to prevent accidental button presses. With this in mind, your case may be covering the proximity sensor, which in turn is turning off the screen. If the above isn't applicable, try heading to Settings > Advanced Features > Motions and Gestures > Keep Screen On while Viewing > On, as this will keep the screen active when you're looking at it, using the front camera to detect your face.
@Gerry Peters: Apologies for the confusion. The proximity sensor is an embedded feature, therefore if you hold the phone up to your ear then the screen will turn off/go into an idle state until you lower the phone. My suggestion regarding the Keep Screen On While Viewing setting was made on the assumption that you would be looking at the phone while using the dial pad. As a workaround, place the phone on Speaker, and avoid raising it to your ear, until all dial pad inputs have been completed.
