[Problem] No incoming sound in calls

For some reason, it started happened a few weeks ago, that when I'm making or receiving a call, they can hear me, but I can't hear anything, and when it happens, I have to restart the phone for it to work. It's like the service making that work was closing itself. Sometimes that even happened while I was in a call, everything was fine, then for a moment I wasn't able to hear anything, and after 10 or 15 seconds I was able to hear again.

What kind of thing can cause that? Is it possible that an app or game or something uses so much ram that it closes such an important service down?