Screen black during phone call and it won't come back during call

I have an Xperia XZs with Android 8. Lately I’m having problems while calling. When I start a call, the screen goes black immediately, and won’t come back anymore during the call. The result is that when I’m in a menu where I need to chose a number, I can’t, because the screen is black. I can’t even hang up. Well, I can pull out the sim card, and that will end the call, but that means a restart.

I’ve installed a new dialer, and that seems to help a little, but the problem is still there.

Is there any way I can get my dialer back to normal? Is this a new bug? I updated recently.

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Update and solution

I see this question is one year old. Back then it wasn’t a big problem, lately it became so bad that I considered buying a new phone, even though this one is good enough for at least a year, maybe two. So I came to the conclusion that the sensor must be broken, and just when I wanted to contact a repair center, I realised that the screen protector may be the problem.

The screen protector has an opening for the proximity sensor. This opening collects dirt along the edges. I removed the dirt, the problem seems gone. It’s maybe to early to be sure, but I’ve done some testing and all seems to work normally. It also explains why things got worse over the course of a year, as the dirt builds up.