noun
extension telephone; extension phone
A telephone connected to an internal line, often used in offices or hotels.
内線電話で部屋までお電話ください。
Please call the room using the extension telephone.
オフィスの内線電話が鳴っている。
The office extension phone is ringing.
内線 refers to the internal line or extension number itself, while 内線電話 specifically refers to the physical telephone set connected to that line.
Compound of 内線 (internal line) and 電話 (telephone).