Step 1: Activate "Filter Unknown Senders"

• Open Settings.
• Tap on Apps and select Messages .
• Scroll down. Under the heading Message Filtering, turn on the switch "Filter Unknown Senders" (this is off by default)
  
  
  Step 2: Find the messages in the Messages app
• Open the Messages app.
• Tap Filters in the top left corner .
• From the menu, choose the option Unknown Senders .
  
  
  
  What if you have an earlier iOS version? (iOS 18)
  If you happen to still use iOS 18, the feature is called "Filter Unknown Senders" and can be found in the same place under Settings > Apps > Messages. It works exactly the same otherwise.