Settings → FaceTime → Blocked Contacts → swipe left and tap Unblock.
Here’s a clear, step-by-step write-up on how to unblock a contact on an iPhone. how to unblock contact iphone
You can also unblock someone directly from the Messages or FaceTime settings: Settings → FaceTime → Blocked Contacts → swipe
Alternatively, you can also unblock a contact through the Settings app. Here's how: how to unblock contact iphone