Hello @Tnorbert, welcome to the community!
I'm sorry to hear about your phone! It must be frustrating!
Have you had the opportunity to locate the proximinty sensor on your phone? On a Xperia z5 Compact it's marked with the number 5 on this picture (clickable link). Start with making sure that the sensor isn't covered by anything, like a case or a screenprotector. Even dirt or dust can make the sensor react, so wipe your screen clean with a soft cloth.
If you experience the same behaviour after cleaning the sensor and making sure that it's not covered by a case or a screen protector, I recommend that you but your phone in safe mode temporarily, just to make sure that no third party application is interacting with your sensor. In safe mode, all downloaded applications are inactivated, but you can still use all your phones basic functions like calling, text messaging and accessing the Internet. Make a call in safe mode, and notice how the phone behaves. To exit safe mode, just restart your phone as usual. If it works fine in safe mode, make sure that all your applications are updated, and maybe uninstall some applications you don't use.
Please keep me updated! I would love to hear how things work out.
