The Australian one will label them as Unverified.

Personally, I'd prefer them to be blocked. If it's important and legitimate, they'll register.

There are good reasons to allow unregistered telecommunications. If you don't like it, you can always block it your end; given that we can all expect many other people to do the same (and it'll likely be a feature built into many phone operating systems), the social pressure you're after still exists.