[noun] [C] - any of two or more telephones in the same house which share the same number, or any of a number of telephones connected to a switchboard in a large building such as an officeWe have an extension in our bedroom.When you call, ask for extension 3276.See also extension at extend.
Meaning of EXTENSION in English
Cambridge English vocab. Кембриджский английский словарь. 2012