My N900 does not identify the number if the caller ID (but not the number) is blocked.

I have my device set to forward calls to my Google Voice number if busy or if the call is rejected, so when I get a call from "Unknown number" and I reject it, it goes there.
Google Voice can, however, identify the number the call was made from, even though the Caller ID for the name is blocked.

I don't care for "unknown" callers and I always reject these calls, but it's strange that the N900 cannot even call back because the phone number is not registered.

Is this behavior a known bug?