Meaning of EXTENSION in English
[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.
Cambridge English vocab. Кембриджский английский словарь. 2012