You can't "screen" a call from an unknown caller - all you can do is set it to reject unknown callers or accept them. (Screening means deciding whether to take the call based on who's calling.)
Um I'm able to screen unknown callers. I do it all the time if I don't recognize the number I press the screen button and it reads of the set dialog. If it's a real person they can respond and I can read what they are saying then take the call if I want.
The call screening actually works really well. One of my favorite things about this phone.