Welcome to the forums, sipotat.
Your device may have a proximity sensor issue. Usually it's the other way around, that is, the screen becomes blank and inactive at unexplained times because the proximity sensor area has become obscured with dust, grease or whatever and just needs to be wiped off to restore functionality.
But it your case it's probably not sensing being held to your head during a call, so it gets hung up, as you say.
I'd try first a reboot, then a live battery pull if the reboot did not remedy the issue, then a factory data reset if those first two things had no effect.
Please post back your results.