Can't hear the other side when calling

I have a BB DTEK 50, Android Version 6.0.1.
Today, I called someone and all of a sudden - after about 30 seconds - I couldn't hear the other person any more.
The normal loud speaker works (in calls, to play music, etc.), the headset works too. Only when I use the phone in a "classic" way right at my ear, I don't hear the other side.
What I already tried:
- I already restarted the phone twice.
- I turned up the volume during the call and reduced it and turned it up again.
- I tried calls via the messenger service "Signal", it doesn't work either.

What can I do?