Business telephone system
The Western Electric 1A family of key telephone units (KTUs) was introduced in the late 1930s and remained in use until the 1950s.
Introduced in 1953, 1A1 key systems simplified wiring with a single KTU for both line and station termination, and increased the features available.
The stations were easier to maintain than the previous electromechanical key systems, as they used efficient LEDs instead of incandescent light bulbs for line status indication.
Early electronic key systems used dedicated handsets which displayed and allowed access to all connected PSTN lines and stations.
Effectively, the aspects that distinguish a PBX from a hybrid key system are the amount, scope, and complexity of the features and facilities offered.
Its intercommunication ability allows two or more stations to directly connect while not using the public switched telephone network.
This method reduces the number of lines needed from the organization to the public switched telephone network.
Initially, PBX systems offered the primary advantage of cost savings for internal phone calls: handling the circuit switching locally reduced charges for telephone service via central-office lines.
As PBX systems gained popularity, they began to feature services not available in the public network, such as hunt groups, call forwarding, and extension dialing.
A PBX differs 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.
A PBX, in contrast to a key system, employs an organizational numbering plan for its stations.
Modern number-analysis systems permit users to dial internal and external telephone numbers without special codes to distinguish the intended destination.
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) differentiated them.
Solid-state digital systems were sometimes referred to as electronic private automatic branch exchanges (EPABX).
[citation needed] The abbreviation now applies to all types of complex, in-house telephony switching systems.
In voice-over IP, hosted solutions are easier to implement as the PBX may be located at and managed by any telephone service provider, connecting to the individual extensions via the Internet.
The upstream provider no longer needs to run direct, local leased lines to the served premises.
As of 2015, ISDN is being phased out by most major telecommunication carriers throughout Europe in favor of all-IP networks, with some expecting complete migration by 2025.
Five distinct scenarios exist: For the option to call from the IP network to the circuit-switched PSTN (SS7/ISUP), the hosted solutions include interconnecting media gateways.
Historically, the expense of full-fledged PBX systems has put them out of reach of small businesses and individuals.
These systems are not comparable in size, robustness, or flexibility to commercial-grade PBXs, but still provide many features.
Functionally, the PBX performs four main call processing duties:[11] In addition to these basic functions, PBXs offer many other calling features and capabilities, with different manufacturers providing different features in an effort to differentiate their products.
