noun
extension phone (and its main line)
Refers to a pair of phones consisting of a main line and one or more extension handsets, often used in a home or small office. The term emphasizes the parent-child relationship between the main unit and its extensions.
親子電話をリビングと寝室に設置した。
I installed an extension phone in the living room and bedroom.
親子電話の子機
The extension handset of the parent-child phone broke, so I bought a new one.