Unlisted number
In telephony, an unlisted number, ex-directory number silent number, silent line, or private number is a telephone number that is intentionally not listed in telephone books.
Silent numbers are often telephone numbers that link to specific tests or control apparatus for the telephone network maintenance staff such as ringback and automatic number announcement circuit and are generally not for public use.
Such numbers are used for residential households as well, primarily for privacy and security concerns.
In Norway the directory services in the 1970s distinguished between secret number, unlisted number and listed number with a hidden address. The first type secret number was typically used by celebrities. The second type unlisted number was not listed in the phone book, but was listed on the directory service. Listed number with a hidden address is useful for women's shelter etc., where the number needs to be listed, but where the address shall 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.
