Don't panic...it is normal. it showing ur proximity sensor is working. 
bro, do you know proximity sensor ? it is integrated on the top of the phone, beside front camera. use to turn off the screen when you make call
When you put your ear on the phone during call. the screen will turn off to avoid any missed-touch to the screen

did you have sp on ur L920 ?
or you can try to run diagnostic tool which hidden in L920.
to activate,
"Press ##634# at the dialer and the tool will open. It will also now appear in your app list."
it can checks screen dead pixel ,proximity sensor and so on.

reminder: the diagnostic tool cant be remove after you activated it.  not sure it will gone or not with hard reset.

or http://forums.wpcentral.com/nokia-lumia-92...uring-call.html

Do you have any casing on the phone that will block the proximity sensor?

My phone is back........ Just need to peel off my screen protector... 

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Added on January 4, 2013, 8:32 am
I have the same problem as you.... i called Nokia support, they told me to peel off screen protector...

The phone function as normal once i peel off..

I found out few possible solution while i having the issue;
1) Remove SP
2) Try wipe the sensor
3) try take few picture with front camera
4) Check whether is that any dust in front facing camera