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In telephony, an unlisted number (United States, New Zealand), ex-directory number (United Kingdom) silent number, silent line (Australia), or private number (New Zealand, and Canada) 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 (and some other countries) 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 (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 70s). 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 to 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.
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