I had a BB storm till about a month ago and I was getting those calls too! I had vzw! It always shows up unknown, only rings for a few seconds then my strom freezes up! I never figured out what it was. I'm with ATT now and have not received any of those calls.
Posted from my CrackBerry at wapforums.crackberry.com
I doubt that is a carrier issue. It is most likely a software related problem. My guess would be the phone is in the process of doing something internally when the call comes in and for some reason that stops the number from coming up.
bear in mind this is just my guess based on almost 9 months of experience using my Storm.