Blocked Calls

If you block a phone number in Settings → Phone → Blocked Calls, the phone number will not be able to contact you unless you delete the phone number from the list yourself. Unfortunately, there is no "block phone number and send it to space" button. Alternatively, you must contact your carrier and ask about your options.

You can find more useful information here:

Block phone numbers, contacts, and emails on your iPhone or iPad - Apple Support

You would have to check with your phone provider to see if they have a Call Block option. If they are not blocked at your service provider, they will go to your phone. Once the call goes to your phone, you only have the Block/Unblock options.