Wiktionary
private branch exchange
n. (context idiomatic English) A telecommunications switching system, physically located at a customer's place of business, that provides internal communication between users and also access to outside (trunk) telephone lines.
Usage examples of "private branch exchange".
The computer has a PBX, a private branch exchange to manage internal phone calls, only like most of the novices in telecommunications, the company doesn't know how to secure the system.