What is the name of the process through which a new subscriber must confirm their subscription before being subscribed to a given list?

The process you described is called double opt-in. It is a two-step verification method used to ensure that a person truly wants to join your mailing list and that the email address or phone number provided is valid and active. How the Double Opt-In Process Works: Step 1 (The Sign-Up): A visitor enters their contact information (email or phone number) into a sign-up form on your website. Step 2 (The Confirmation): An automated message is immediately sent to that address or number. This message contains a link or a button (or a keyword for SMS) that the user must click or reply to. Completion: Only after the user performs this second action are they officially added to your list as an "Active" subscriber.