unblock "no caller id"

I blocked someone who called with "no caller id" and need to unblock them. I know the block is active because they are going straight to voicemail and it's telling them the mailbox is full.

I cannot find a way to unblock this number in Settings>phone>Call Blocking & Identification. The number is not in the list under Blocked Contacts.

Short of unblocking my whole list of robocalls, is there another way to allow this person to call through? Could I add them to my contacts to work around the block?