[JSR_FDED]

Singapore does this. Any message that comes from an unregistered sender show up on the phone with “Likely Scam” as the sender name.

[steve_taylor]

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.

[wizzwizz4]

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.