A telephone switching system used within an organization to connect internal extensions and external lines.
The company's PBX broke down, and outside lines became unavailable.
After introducing a new PBX, the quality of internal calls improved.
A general telephone exchange; 構内電話交換機 specifically refers to a private branch exchange used within a single organization.
Compound of 構内 (premises), 電話 (telephone), and 交換機 (exchange). The term is a direct translation of the English 'private branch exchange'.