Private Branch Exchange. It enables a sharing of telephone lines by extensions through an operator, not automatic, even for outgoing calls.
Your own, smaller version of the phone company's central switching office. A PBX controls all your incoming and outgoing calls, and offers the benefits of many automated features, easy set up and flexibility when adding or changing your phone system.
Private Brunch eXchange
Manually operated telephone switchboard. Literally a patch board exchange. The Battery Control Van (IFC area) has one, as does the Launch Control Van.