Screen turns off on call, no sound from ear speaker

Hi,

My dad's phone (LG G2 D802) got wet and I managed to replace it's screen (the entire display assembly).

After the replacement, the phone turns on and the screen work just fine.

The problem is that when answering an incoming phone (or calling), the screen turns black and you cannot hear the caller's voice (you can hear the sound from the mic on the other side though).

I think I may have damaged the ear speaker. I will replace it when getting the part. But I cannot get why the screen is going black? Is it related somehow?

Any ideas may help.

Thank you!