@GuzziBoy: Our Galaxy mobile devices have a proximity sensor which will switch off the screen when the phone is held to your ear. This is to prevent accidental button presses while on a call, and cannot be disabled in the phone's settings.
As a workaround, please head to Settings > Lock screen and AOD > Extend unlock, and follow the on-screen prompts. This keeps the phone unlocked while you're holding it, when you're near home or another trusted place, or your watch or another trusted device is nearby. I'm not certain if this will override the proximity sensor, but it is worth exploring as an option. It will certainly be of use to you if you place the call on 'Speaker', and the phone is away from your face.
