Hi @Me02, let me see if I can help out here!
There is a sensor at the upper part of your display (see point 4 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 test the proximity sensor in the support app. Find and tap Settings > Support > Xperia Tests > Proximity sensor
Try this and let me know how it goes!
