phone update 
The Nokia shop could not help me with the mis-feature I think the phone has - they told me, the behaviour is on purpose (The main navigation button switches to loud-speaker on call acceptance).
But they helped me to open the phone. The knob does not need to be pressed deeply, you just must forcefully move the cover to the top of the phone. That is hard to do as you need to grab on something, so one thumb is at the knob, the other below the camera lens - and you need dry hands. (Do not do it often, as the cover will 'wear out')