Try to force reboot your smartphone this will solve your problem. https://okadtech.com/sony-xperia-z5-black-screen-of-death-problem-solution/ Check it this will help you in solving this problem.

Do these things first (1) Reboot your smartphone. (2) Update your smartphone. (3) Clear cache of phone app. If these 3 things doesn’t work and you are still having call related issue or wifi calling issue then try to reset your network settings this will solve your problem. You can refer this article How to fix motorola call/ wifi calling problem Thanks