PBX Private Branch Exchange Telephone Systems: “Invisible Diplomats” 1965 AT&T; Audrey Meadows

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AT&T high-end business marketing film for PBX (Private Branch eXchange) switchboards. The film stars Audrey Meadows & Ruta Lee, the cinematographer was Hal Mohr (A Midsummer Night’s Dream, The Phantom of the Opera, Rancho Notorious…).

Two different kinds are shown:

Audrey Meadows’ PBX is a more traditional cord switchboard. The operator answers and connects calls by plugging cords into jacks.

Ruta Lee’s PBX is the latest (in 1965) cordless switchboard. Calls are processed by pressing a sequence of buttons, instead of using cords and jacks.

Public domain film from the Prelinger Archives, slightly cropped to remove uneven edges, with the aspect ratio corrected, and mild video noise reduction applied.
The soundtrack was also processed with volume normalization, noise reduction, clipping reduction, and/or equalization (the resulting sound, though not perfect, is far less noisy than the original).

A private branch exchange (PBX) is a telephone exchange or switching system that serves a private organization and performs concentration of central office lines or trunks and provides intercommunication between a large number of telephone stations in the organization. The central office lines provide connections to the public switched telephone network and the concentration aspect of a PBX permits the shared use of these lines between all stations in the organization. The intercommunication aspect allows two or more stations to establish telephone or conferencing calls between them without using the central office equipment.

Each PBX-connected station, such as a telephone set, a fax machine, or a computer modem, is often referred to as an extension and has a designated extension telephone number that may or may not be mapped automatically to the numbering plan of the central office and the telephone number block allocated to the PBX.

Initially, the primary advantage of a PBX was the cost savings for internal phone calls: handling the circuit switching locally reduced charges for telephone service via the central office lines. As PBX systems gained popularity, they were equipped with services that were not available in the public network, such as hunt groups, call forwarding, and extension dialing. In the 1960s a simulated PBX known as Centrex provided similar features from the central telephone exchange.

A PBX is differentiated from a key telephone system (KTS) in that users of a key system manually select their own outgoing lines on special telephone sets that control buttons for this purpose, while PBXs select the outgoing line automatically or, formerly, by an operator. The telephone sets connected to a PBX do not normally have special keys for central office line control, but it is not uncommon for key systems to be connected to a PBX to extend its services.

A PBX, in contrast to a key system, employs an organizational numbering plan for its stations. In addition, a dial plan determines whether additional digit sequences must be prefixed when dialing to obtain access to a central office trunk. Modern number analysis systems permit users to dial internal and external telephone numbers without special codes to distinguish the intended destination.

History

The term PBX was first applied when switchboard operators managed company switchboards manually using cord circuits. As automated electromechanical switches and later electronic switching systems gradually replaced the manual systems, the terms private automatic branch exchange (PABX) and private manual branch exchange (PMBX) were used to differentiate them. Solid state digital systems were sometimes referred to as electronic private automatic branch exchanges (EPABX). Today, the term PBX is by far the most widely recognized. The acronym is now applied to all types of complex, in-house telephony switching systems.

Two significant developments during the 1990s led to new types of PBX systems. One was the massive growth of data networks and increased public understanding of packet switching. Companies needed packet switched networks for data, so using them for telephone calls was tempting, and the availability of the Internet as a global delivery system made packet switched communications even more attractive. These factors led to the development of the voice over IP PBX, or IP-PBX.

The other trend was the idea of focusing on core competence. PBX services had always been hard to arrange for smaller companies, and many companies realized that handling their own telephony was not their core competence. These considerations gave rise to the concept of the hosted PBX…