To unblock someone on iPhone, go to Settings > Phone > Blocked Contacts. Find the person you want to unblock, swipe left on their name, and tap Unblock. They can now call, message, or FaceTime you again.
To unblock someone on iPhone, open Settings, go to Phone, Messages, or FaceTime, then tap Blocked Contacts. Find the number or name you want to unblock and swipe left or tap Edit, then Unblock. The person will be able to call, text, and FaceTime you again.
Navigate to Settings > Phone (or FaceTime or Messages). Select Blocked Contacts (or, on older iOS, Call Blocking & Identification). Locate the individual, tap the red Unblock button, and swipe left on the name or number. You can also unblock them via their Contact Card.