Thanks Rich for your prompt reply. Below the answers to your questions.

I'd like to confirm, did you mean factory data reset?

Yes, complete factory reset.

If so, did you restore your apps right after doing it?

Yes, I did, using Android default restore functionality (even icon positions were restored)

When you receive a call and cannot reject it, what activity you were doing with your phone?

I've not associated to a specific activity. It already happened while the phone was in my pocket and also when I was looking to home screen (during some tries to solve the problem). It is important to observe that I can't reject incoming calls but also can't stop calling someone! The same problem happens when using Android Auto, so it is probably not an interface stuff. 

Have you tried pressing the power button and try to reject it? 

No - I will try next time. Do you mean a single short press or a long press? Is it supposed to cancel the call?

Please update your profile with the current phone, carrier and serial number -- this will help me investigate.

Done.

Another tests and things I've noticed since this post:

1. Removed permissions from bank apps and other unimportant apps to access the telephone. No success.
2. Tried installing another dialer (no success as the calls need the default phone app)
3. I've noticed that swiping the phone app up on the app switching screen after the caller (or me, when I originate the call)gave up makes the phone app go back to the contacts/dialing screen as usual - however there are some internal process still going up as I can't send voice messages through whatsapp or originate another call (in the later case pressing the green call button is innocuous)

Will it help if I record my screen during the problem and attach here?

Thank you very much, again!