Ting can do it. It's one checkbox in my device settings panel. Here's the whole list of things I can tweak:

> Can use call forwarding > Can use call forwarding on busy/no answer > Can use call forwarding at any time > Can use call waiting > Can use hold > Can use three-way calling > Can use visual voicemail > Can make/receive calls > Can use voicemail > Can use Wi-Fi calling > Allow international long distance calls > Allow international roaming > Allow international roaming data > Can send/receive text messages > Block picture, video, and group messages > Can use data > Can use tethering

If you go to Settings > Messages, you can filter out messages from unknown senders. However, this only prevents the notifications from showing up; they'll still be in the app under the "Unknown senders" tab.

FWIW, T-Mobile does allow you to block all SMS and MMS.

There is also a Screen Time feature called Communication Limits which can be used to restrict Messages and Phone to contacts only. Doesn't apply to third-party apps though.

On Android you could deny SMS access to Google Play Services and/or the Messaging app.

I'm not suggesting that you switch, only that something similar might exist on iOS.

Not that you should have to, but look into Nomorobo, which can help with robocallers, and spam text messages.

Can you add yourself/spouse to the addressbook, and then block messages from unknown senders?

> I spent over an hour on the phone, they weren’t able to do it.

They are. They just refuse to.

Why?