How do you block Unavailable calls

You can't block unavailable calls using phone company blocking service. You'll need a 3rd party blocking device or a phone with smart call blocker. 

Another thing that AT&T can’t or won’tdo.

They are governed by the FCC as to what they can and can't block. Unavailable means no data was sent so it could be a glitch and if they block all unavailable they could block legitimate calls. If you use one of the methods suggested than it's you not the phone company that's responsible if a legitimate call gets blocked. You don't have to agree with it, but that's the way it is.