nandohyphen1 An idea that comes to mind is that cause might also be software. If "proximity sensor turning off the screen during a call" is considered a new context change inside the Phone app, but the developers simply forgot to add a wake-lock into it, it'd explain why after being on a call for a while the screen would not turn back on by itself. It simply has had enough time to lock itself while being kept dark by the proximity sensor.
I've noticed this behavior myself, too. When I'm in calls, if they're really short, the sensor works and the screen turns back on when I pull it away from my ear; but in longer calls it stays dark after being pulled away.
But this is just an idea. I haven't checked the code and I haven't made any tests with it; maybe I should in the future.