Dial phone, screen blanks -> can't hang up
Please check if it happens in safe mode too:


If the problem is not resolved, factory-reset is next step.
If it is resolved, it implies one of the installed apps is the culprit. First step could be to remove all apps having telephony related permissions and then check. Add apps one by one and check. Painful process, yes, or go nuclear and factory reset after backing up your data.