This is commonly-misunderstood behavior, unique to Google Voice. When the automated voice says "call from unknown caller", what it actually means is, "call from somebody whose name and telephone number are not already in your Google Voice contacts, so I can't look it up and tell you who is calling." It is not the same as getting a call from a telephone number with blocked or missing caller ID number or caller ID name.
So: if the caller is somebody you converse with often, then add their info to your Google Contacts. Or, turn on call screening with the additional option to ask the caller to speak their name, and it will be recorded. Beware: that feature is annoying to some callers, and, if the caller doesn't actually say their name, but instead, says, "um..." or "Kim Kardashian" or "I don't want to tell you my name", that will be forever played back to you, with no way to change it.
