Microsoft is pretty notorious for bugs like this. Like Windows, there's usually no workaround while we wait for fixes.
Why not make this your reason to ask for a full refund and try another phone?
Bob
after updating my Lumia 930 to denim update version 02540.00019.14522.26003 I get a big problem during call. yes my phone screen get blacked and I can't use it normally.
For many days I'm looking for why my phone display get black during call and after searching and doing too many ways, I decide to make a reset factory on my phone and step by step configuring and testing call to find problem, after doing reset factory there isn't any problem until that for first time I touch device hub from device setting and it show that for some updates it need to restart my phone, after restarting the black screen problem during call came back and I found the problem is depend on device hub and its updates.

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