iMessage's "Block Caller" option doesn't stop carrier spam texts

I'm on an extended trip to Southeast Asia, and I'm using a local carrier with my iPhone. My carrier is sending me spam via text messages several times daily (occasionally also via Government Alerts). The Messages app's option to block a caller doesn't work for the various senders involved. Although the option is available, it's reset immediately after I leave the contact card within Messages. The option to hide alerts works just fine, so it looks like Messages is able to identify the senders, i.e., the carrier isn't just using a unique number or name for each text message. None of the senders have phone numbers associated with them, so I can't just add the numbers to a blocked contact within the Contacts app. Does anyone have a solution or any insight into why Messages doesn't block the senders? Here's a [screen recording](https://www.youtube.com/watch?v=pTs4jPYLIrI) and a screenshot of the "Block Caller" option: