Hi @Susie68, let me see if I can help out here!
There is a sensor at the upper part of your display (see point 7 in this overview) which detects if your ear is within proximity or not, and if it is then your display will go dark during calls. This is to avoid you accidentally ending calls with your cheek for example.
Do you have a display protector mounted on your device, or any cover which could cover the sensor? Try removing it in that case to see if the behaviour persists. You can also test the functionality of the proximity sensor in the support app. Find and tap Settings > Support > Xperia Tests > Proximity sensor.
Another thing that you can try is to reboot your phone in safe mode to see if the proximity sensor works better there. All apps downloaded from Play Store is temporarily deactivated while in safe mode and by doing this, you can see if this behaviour is caused by a downloaded app. For more information on safe mode and how to use it, click here.
Try this and let me know how it goes!
