I try my best to help even though I can only barely comprehend what you said. Correct me if I've misunderstood your situation.

During phone rings, the screen will go dark when something is getting too near the screen, e.g. your face. That's to say, when the phone is ringing in your pouch the screen should remain locked, and unlock automatically when nothing is blocking the screen.

Also, the phone is locked again after hanging up. It should not remain unlock even when you failed to pick up the call.

That's to say, your situation is definitely abnormal. There are two things I'd like to check with you:

1) I suspect that your proximity sensor is not working. Try calling yourself again and block the proximity with finger, to see if the screen would go dark as normal
2) What is your firmware version? Newer firmware has fixed a lot of issues associate with phone app, so I wonder if you're still using the older firmware.