Do the sensor test. I got a refurb this week that had a faulty sensor and would go black as soon as I dialed and would not wake up without sliding open...and sometimes required a battery pull.
Just got my refurb phone cause the first one (bought on day one 11/5/09) died. The refurb screen goes black as soon as the call is connected and won't come back unless I slide it open or pull battery. I called Verizon tech support and they're sending another refurb to me. Oh joy another one of these POSs!
My phone had this problem, then I decided to peel off the screen protector. The screen protector blocks this sensor... When it was on, it would go blank and I had to hit the power button to access the dialpad. I took it off, it works properly. I have a mirrored screen protector, maybe the transparent one won't do this. For now, I have replaced the protector, but plan to replace it with a clear one.