[2] In 1971 The New York Times wrote that some users of this service needed it to hide from bill collectors.
The first type secret number was typically used by celebrities (in this case the address was hidden as well).
The second type unlisted number was not listed in the (paper based) phone book, but was listed on the directory service (a voice call to 018 in the 1970s).
Listed number with a hidden address is useful for women's shelter etc., where the number needs to be listed, but where the address is to be hidden from the general public.
Naturally, there are many changes when directory services became available on the Internet, but this case shows that separate user groups may have different needs to hide various parameters relating to privacy.