In telephony, a silent number, unlisted number (Canada, United States, New Zealand), ex-directory number (United Kingdom) or private number (Australia) is a telephone number that is intentionally not listed in telephone books.
Silent numbers often exist as certain telephone numbers that link to specific tests or control apparatus for the telephone network maintenance staff (such as ringback) and are generally not for public use.
Silent numbers are used for residential households as well, primarily for privacy and security concerns.
In Norway (at least) the directory services (in the 70s and probably still) separated 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 of course hidden as well). The second type unlisted number was not listed in the (paper based) phone book,was listed on the directory service (a voice call to 018 in the 70s). Listed number with a hidden address is useful for womans'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