Unknown number instead of anonymous number

I've got a question, sometimes when the Caller ID is hidden, it will show up on my lock screen as "Anonymous". But recently, I've noticed something, there is a distinction between Anonymous and "Unknown", when missing a call. Who can tell me what "unknown number" means, I suppose a caller either has the number hidden (anonymous) or shown (just the number that shows up, or a contact)? I also received a voicemail from the unknown number, which is just a 3 second message with a computer voice. Who can clear this up for me?