Hi @S02, welcome to our forum and I'm sorry to hear about this.
Do I understand correctly that when you call a person and before you have taken the device up to your ear the display turns off by itself but you can still talk to the person that you are calling? If that is the case it does sound like the proximity sensor on your device is not working correctly as the display should only turn off when the sensor is covered, for example by your ear.
On the link below you can see where this sensor is placed (Number 1). Make sure that you don't have a case or a screen protector that is covering this sensor. You can also try using a dry cloth to clean the top part of your display.
https://support.sonymobile.com/global-en/xperiac4/userguide/overview---cos/
I would also recommend that you try to restart your device in safe mode to make sure that no downloaded application is the cause of this behaviour. While the device is rebooted in safe mode, place a call or do the inbuilt test in the device that you can find in Settings > About phone > Diagnostics > Test > Proximity sensor.
Let me know how it goes! 
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