Why does my screen go dark and remain so, even when I receive a call?

I have set my screen time out at 5 minutes. After that, the screen goes dark and remains so even if I receive an incoming call. This prevents me from seeing who the call is from or even responding to it! The screen will come back if I "tap" the on/off button, but this wipes out the call. Sometimes the screen will come back if I strike the side of the phone.

I can't find anything in the user manual to address this problem. Can anyone help?

Many thanks,

Ed