Tap Blocked Contacts.
For devices running iOS 12.4 or lower, tap Call Blocking & Identification.
Tap Add New... then enter preferred contact(s) by typing in search box or scrolling through the list.
For devices running iOS 12.4 or lower, tap Block Contact.. then enter the preferred contact.
To unblock a contact:
Tap Edit (upper-right).
Tap the red minus icon to the right of the preferred contacts.
Tap Unblock to confirm then tap Done (upper-right).
For additional blocking options, check out these FAQs.