Bug check - report here if you are seeing persistant screen blanking in call.
On my Nokia Lumia I am seeing one fairly fatal flaw... when I receive a call, the display blanks as soon as I answer the call. Pressing the power button will very briefly flash the screen on, then it immediately blanks again.
It's as if the proximity sensor is "always on". Whatever it is, it makes it impossible to use "phone tree" "press 1 for..." kinda navigation, or transfer a call to/from speaker or BT, or use the phone while talking.
To test this, let me know if you have a functional display when in-call
1) when you've received a call
2) when you place a call
If you have to press power to wake up the phone display or not, that would be good to note too.
If it's working for everyone else, then my phone's got a hardware fail (proximity sensor is stuck/dead)