Blocking 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

Sophiakam wrote:

The blocking function on iPhone seems to be nothing more than a way to send a call straight to voicemail. Is there a way to truly block someone such that they can’t leave you messages? It kind of defeats the purposes of the block.

The messages are in a "Deleted" folder so you can easily avoid ever seeing them. Why does it defeat the purpose? Allowing the person to leave the voicemail in fact serves an important purpose: it prevents the caller from knowing they have been blocked. The last thing Apple wants is to hear that someone found out they were being blocked and became violent.

If you want to truly block a number so that it never gets to your phone, you'll need to see what you're carrier offers. Usually, they charge for more than a couple of numbers.

The only way to truly block them so that they can't leave a message would be to do that at the carrier level.

All the iPhone, or any phone for that matter, can do is reject the call without ringing.