A private automatic branch exchange (PABX) is an automatic telephone switching system within a private enterprise. Originally, such systems – called private branch exchanges (PBX) – required the use of a live operator. Since almost all private branch exchanges today are automatic, the abbreviation “PBX” usually implies a “PABX.”
   A PBX (private branch exchange) is a telephone system within an enterprise that switches calls between enterprise users on local lines while allowing all users to share a certain number of external phone lines. The main purpose of a PBX is to save the cost of requiring a line for each user to the telephone company’s central office.