I can confirm it is definitely not a software issue, nothing to do with apps or reset etc. I had this issue on a z5 premium. (phone 1) Stopped using it and I later bought another phone exactly the same (phone 2) which did not have this problem. Unfortunately I dropped it cracking the screen. So I took the motherboard out of phone 2 and put it in phone 1. It wasn't very long before I remembered why I dumped phone 1 because it started going black during calls again. This is why I can't conform it Is hardware related. Now I just have to work out what part of the internal bits is causing this.
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