Private Branch Exchange (PBX)

Private Branch Exchange Or PBX An Overview Of Business Telephone System

A business telephone system is a multiline telephone system typically used in business environments, encompassing systems ranging in technology from the key telephone system (KTS) to the private branch exchange (PBX). 

TESLA Automatic system Telephone Exchange model in the early 1970s. (Source: WikiPedia)

A business telephone system is different from multiple central office (CO) telephone installations because the CO lines utilized may be directly controlled by key telephone systems at many telephone stations and frequently provides extra characteristics for the processing of the call. Business telephone systems are generally widely categorized into key telephone systems and private sector exchanges; nevertheless, there are numerous hybrid systems. 

A key telephone system originated from a private branch exchange since the central office and station connections were not requested by an operator or attendant at the switchboard. Technologically, a central office system can compete in capacities and features with shared lines of private branches, with the central office telephony systems, and with bigger and more complicated systems. A station user can directly regulate the connection with a key telephone system by means of line buttons indicating the condition of the lines with built-in LEDs. 

Western Electric Telephone Set Early Model 464G (Source: Wikipedia)

Key business telephone system or Key telephone system

Key telephone systems are defined largely by configurations for each telephone line that is accessible with separate selection buttons. The early business telephone systems were called cable systems. They comprised merely of telephone sets, keys, lighting, and cables. 

Key was a Bell System art word for a customer-controlled switching system such as the line buttons on telephones. 

In the first 1A key telephone system established in the 1930s at the Bell System, these cabling designs grew into modular hardware components with a wide range of capabilities and services. 

Three main designs may be used to build key systems: electromechanical shared control, electronic shared control, or independent key sets. 

The hybrid systems and equivalent-sized private branches exchanges have equal costs and more functionality. New installations of the main phone networks are less prevalent. 

Key telephone for private branch exchange from the 1930s (Source: WikiPedia)

Electromechanical shared-control key system 

Key business telephone systems were normally built of electromechanical (relays) components, as were major telephone switching systems, until the introduction of massive integrated circuits. 

Typical examples and offered for decades in North America are the systems 1A, 6A, 1A1, and 1A2 Key System. Initiated in the late 1930s and in the 1950s, the 1A family of key telephone units (KTUs) from the Western Electric Company (WECo). 1A device was rudimentary and needed two KTUs per line, one for the termination line and one for the termination of the station (telephone device). The WECo 300 series telephone was the telephone device used by 1A systems. Launched in 1953, one-way KTU cabling for both line and station termination is simplified and functionality is improved with 1A1 key systems. The need for intercom functionality rose as the 1A1 systems became popular. For a single speak connection, that is, a single discussion at a time, the initial intercom KTUs, Weco Model 207 were connected. The WECo 6A dial intercom has two speakers and was frequently fitted in a 1A1 or 1A2 key system as a dial intercom. The 6A systems were complex, difficult and costly, and never popular. With the emergence of 1A2 in 1964, the essential system installation and maintenance were simplified. The electronic systems, with their faster installation and higher functionality, continued to be utilized into the 1980s, when electromechanical systems signal the end. 

Two lesser-known key communications systems, the 102 and 302 key systems, were utilized at airports. They were built specifically for communication between the RAPCON and the GCA, including radio line connections, air traffic control tower, and radio approach control. 

The Automatic Electric company also manufactured a series of key telephone devices, some of which were Western Electric compatible, but the widespread usage of Western Electric devices was not gained. 

Electronic shared-control system 

The same design may be used cheaper with the arrival of LSI ICs than relays were conceivable. Moreover, the multi-wire wiring may be eliminated and replaced with much simpler cables like or comparable to the non-key cable cables. Shared electronic systems rapidly evolved to the present hybrid phone system, as PBX and key system function combined swiftly. The AT&T Merlin is one of the best-known systems. 

These more advanced technologies have also permitted a range of functionalities, including: 

  • Answering machine functions 
  • Automatic call accounting 
  • Caller ID 
  • Remote supervision of the entire system 
  • Selection of signaling sounds 
  • Speed dialing 
  • Station-specific limitations (such as no long distance access or no paging) 

Features that allow simple modification of these systems might be readily updated or added utilizing the software. The stations were easier than earlier electromechanical key systems to maintain since they employed efficient LEDs for a line status signal rather than incandescent light bulbs. 

LSI also allowed smaller systems, which did not require a shared control unit, to delegate controls (und functions), into individual telephone sets. Generally, these systems are utilized with relatively few telephone sets, and the functionality set (sample speed dialing numbers) is frequently more difficult to keep synchronized among the different sets. 

Hybrid key business telephone system 

The line between key systems and PBX systems has grown more and more blurred over the 21st century. Early electronic key-systems utilized specialized handsets to show all linked PSTN lines and stations and to enable them access. 

Now, the contemporary key system offers a range of capabilities more commonly seen in big business telephone systems or PBX systems, such as SIP, ISDN, and analog phones. In addition to its own unique handsets – generally digital. The hybrid label comes from the support both of analog and digital signals and from some PBX functionalities. 

A hybrid system usually includes certain call appearance buttons, which match particular lines and/or stations directly but also enable direct dialing to extensions or external lines without choosing a line appearance. 

The current key system is generally completely digital, although if analog variations continue to exist and certain systems use VOIP services. In fact, the features, scope, and complexity supplied are the factors that separate a PBX from a hybrid key system. 

In the financial services business, hybrid systems are a frequent tool utilized on commercial floors. These sophisticated hybrid key systems typically just need associated PBXs for back-office and voicemail interaction. These systems usually have their front-end units called Turrets and are known for their hoot-n-holler circuits. Many hoots are shown to several users at various locations using multiplexed speakers. 

Private branch exchange or PBX

MiVoice MX-ONE is a full-featured communication server PBX that supports analog, digital, IP communication and is one of the most famous Business Telephone Systems from Mitel

A PBX is a telephone exchange system or switching system which is used to service a private organization and allows central office trunks to be shared amongst in-house phones and allows intercourse between these in-house telephones without the need of external lines. Connections to the public switched phone network (PSTN) are provided by the central office lines, and the concentration aspects of a PBX allow these lines to share across all stations in the company. Its ability to connect two or more stations directly without utilizing the public telecommunications network. This means that the number of lines required from the organization to the public telephone network is reduced. 

Each device linked to the PBX is referred to as an extension and has a specified extension number, such as telephone, fax machine, and computer modem, that may be automatically mapped to the Central Office Numeration Plan and the PBX telephone number block. 

The major advantage of cost savings for internal phone calling was first given by the PBX systems: the local management of circuit switches lowered telephone service rates via the central office lines. As PBX systems became more popular, features such as search groups, call forwarding, and extension dials were launched not available on the public network. A comparable functionality from a central exchange was offered by a simulated PBX from the 1960s known as Centrex. 

A PBX varies from the Key Telephone System (KTS) because a key system user selects its own outgoing lines using specially designed telephones, which control buttons to do this, while PBXs automatically choose the outgoing lines. The telephone sets that are connected to a PBX usually have no specific key for central office line control, but to extend the services to key PBX systems is not unusual. 

A PBX uses a numbering strategy for its stations, unlike a key system. A dial plan also decides if extra number sequences need to be prefixed to access a central office trunk while calling. Users can call internal and external telephone numbers without specific codes to determine the intended destination via modern number analysis systems. 


The name PBX was derived from the manual use of cord circuits by switchboard operators. The names Private Automated Branch Exchange (PABX) and private manual branch exchange (PMBx) differed as automated electromechanical switches and subsequently, electronic switching technologies increasingly superseded the manual systems. Digital sound systems have occasionally been described as automated Electronic Private Branch Exchanges (EPABX). The word PBX is by far the most well-known from 2016 forward. The term currently covers all sorts of in-house sophisticated telephone switching systems. 

During the 1990s, two major breakthroughs led to new forms of PBX. One is that data network networks are massively growing and packet change has become more public. Companies required packet-switch networks for data, which made use of them for phone calls more desirable and made packet-switching communication even more attractive by the availability of the Internet as a worldwide delivery method. These reasons have led to voice growth via IP PBX or IP-PBX. 

The opposite trend was to concentrate on core expertise. For smaller firms, PBX services had long been difficult to organize and many companies concluded that controlling their own telephones was not their primary skill. The notion of the host PBX was created by these concerns. The Centrex service supplied by telcos in wireline telephony since the 1960s has been the first host PBX, which later became a competitive contemporary local exchange career. Host solutions are easy to deploy in voice over IP, since the PBX may be placed at any telephone services provider and controlled by connecting them over the Internet to their particular extensions. The upstream supplier must no longer run to the serviced premises on direct, local rental lines. 

Manual PBX 

A call operator connects manually to a telephone switchboard with chord couples

Many manufacturers have supplied manually controlled private branch exchange systems in different sizes and functions; here are some examples: 

System components 

A PBX often includes: 

  • Cabinets, closets, vaults, and other housings. 
  • The operator can control incoming calls using a console or a switchboard. 
  • Interconnecting wires and cables. 
  • Logic cards for PBX operations, switching and control cards, power cards, and associated equipment. 
  • For arbitrary data processing, control, and logic, a microcontroller or microcomputer. 
  • Exterior telco trunks that provide (and transmit) signals to the PBX. 
  • Stations or phone sets called lines occasionally. 
  • The PBX’s internal switching network. 
  • The UPS consists of sensors and power switches, as well as batteries. 

Current trends 

The development of PBX has been targeted towards the IP PBX, which is based on the Internet Protocol. Since the advance of Internet telephony (Voice over IP) technologies. Most contemporary PBX support VoIP. VoIP support. ISDN PBX systems superseded certain older PBX systems in the 1990’s because ISDN includes conference telephone, call forwarding, and configurable caller identification capabilities. In 2015, ISDN being phased out in favor of all-IP networks, with some anticipating complete migration by2025 by most of the leading telecommunications operators across Europe. In the beginning, PBXs have been VoIP Centres, housed by the operators or even manufacturers, and have been first used as manual switchboards or assistive consoles for a telephone operator or just for the operator. 

Even though many people believe VoIP to be the future of telephony, the switched-circuit network continues to be the core of communication. In services with current IP systems, old PBX systems remain competitive. There are five separate scenarios: 

  • Hosted/virtual PBX (hosted and circuit-switched) or traditional Centrex 
  • IP Centrex or hosted/virtual IP (hosted and packet-switched) 
  • IP PBX (private and packet-switched) 
  • Mobile PBX solution (mobile phones replacing or used in combination with fixed phones) 
  • PBX (private and circuit-switched) 

The hosted solutions feature interconnecting media gateways for the option of connecting from the IP network to the circuit-switched PSTN (SS7/ISUP). 

Home and small-business usage 

In the past, the costs of complete PBX systems have placed small enterprises and people outside their grasp. Since the nineties, however, there have been several smaller, consumer, and consumer PBXs available. These systems are not comparable to commercial-grade PBXs in size, robustness or flexibility, but nevertheless have numerous functions. 

Analog (POTS) telephone lines were the first consumer PBX systems that handle four private analog and one public analog line. It was the size of a tiny box of cigars. In Europe, these analog telephone networks have been succeeded by the ISDN business telephone systems. The logical step of using tiny ISDN PBXs or ISDN business telephone systems is that the ISDN base rate interface has two logical telephone lines, which may be utilized in tandem through two ISDN B channels. When consumer VoIP adoptions were implemented, consumer VoIP PBXs became simple, extra software features on consumer-grade routers and switches with the PBX capabilities. In addition, several providers of telecommunications now provide Host PBX systems where the PBX is hosted by the provider and connects to the phone handsets through internet access. 

Since the 1990s, open-source project PBX-style features have been available. These projects provide flexibility, functionality, and programmability. 

IP-PBX started in 1997 and took the next level of corporate communication. In order to manage the routed and switched calls, IP-PBX is a PBX phone system that employs IP data networks to handle messages. Virtual hosting has resulted in considerable cost savings. In addition, it may also utilize the VoIP gateways to connect to the regular PSTN line, as well as sophisticated services (such as voicemail). This allowed the user to continue with the same carrier. In addition to improving cost advantages and user interface over time, the use of IP-PBX in smaller enterprises has grown. 

PBX functions 

PBX carries out four primary call processing tasks functionally: 

  • Connections (circuits) between the two users’ telephone sets (e.g. maps a called telephone numbers so that the phone isn’t already operating). 
  • Keep connections as long as the users need them (i.e. channeling voice signals between the users) 
  • Disconnect these links as required by the user 
  • Providing accountability information (e.g. metering calls) 

PBXs provide many more calling capabilities, as well as these fundamental tasks, and various manufacturers offer different functionalities so their systems may be differentiated from one another. Include common functions (for each capability, producers might have a different name): 

  • Auto-attendant 
  • Auto dialing 
  • Automated directory services (where callers can be routed to a given employee by keying or speaking the letters of the employee’s name) 
  • Automatic call distributor 
  • Automatic ring back 
  • Busy override 
  • Call blocking 
  • Call forwarding on busy or absence 
  • Call logging 
  • Call park 
  • Call pick-up 
  • Call transfer 
  • Call waiting 
  • Camp-on 
  • Conference call 
  • Custom greetings 
  • Customized abbreviated dialing (speed dialing) 
  • Direct inward dialing (DID) 
  • Direct inward system access (DISA) (the ability to access internal features from an outside telephone line) 
  • Do not disturb (DND) 
  • Follow-me, also known as find-me: Determines the routing of incoming calls. The exchange is configured with a list of numbers for a person. When a call is received for that person, the exchange routes it to each number on the list in turn until either the call is answered or the list is exhausted (at which point the call may be routed to a voice mail system). 
  • Interactive voice response 
  • Local connection: Another useful attribute of a hosted PBX is the ability to have a local number in cities in which you are not physically present. This service essentially lets you create a virtual office presence anywhere in the world.[10] 
  • Music on hold 
  • Night service 
  • Public address voice paging 
  • Shared message boxes (where a department can have a shared voicemail box) 
  • Voice mail 
  • Voice message broadcasting 
  • Welcome message 

Interface standards 

PBX extension connection interfaces include: 

  • DECT – The cableless phone connection standard. 
  • Internet Protocol – For example, H.323 and SIP. 
  • POTS (plain old telephone service) – Used in most houses as a standard two-wire interface. This is cost-effective and may be used as an extension by nearly any basic phone. 
  • Proprietary – A procedure has been specified by the manufacturer. The sets of manufacturers can only be connected to their PBX, but more visible information and/or function buttons are presented. 

Connection interfaces between PBXs include: 

  • DPNSS – For trunk lines linking PBXs. This is generally done using physical circuits E1 (E-carrier), standardized by British Telecom. 
  • Internet Protocol – The IP-based options for multimedia sessions include H.323 and Session Initiation Protocol (SIP). 
  • Primary rate interface (ISDN) – Supplied by T1 (23 channel bearers and 1 channel of signals) and E1 carriers. 
  • Proprietary protocols – The adoption of a common procedure is necessary if equipment from many manufacturers is on site. 
  • QSIG – Usually, T1 (T-carrier) or E1 (E-carrier) physical lines are operational for linking PBXs to one other. 

PBX or business telephone system connectivity interfaces to trunk lines include: 

  • Internet Protocol – The IP-operated protocols H323, SIP, MGCP, and Inter-Asterisk eXchange are supported by several network operators. 
  • ISDN – Fixed telephony devices’ most prevalent digital standard. This is available in either Basic (2-circuit) or Primary variants (24 or 30-circuit). Most medium to big businesses would have had physical primary ISDN connections on T1 and E1. 
  • RBS (robbed bit signaling) – Over a four-wire interface (T1) produces 24 digital circuits 
  • Standard POTS (plain old telephone service) lines – Used in most houses, the typical two-wire interface. This is only suitable for smaller systems and cannot be detected while trying to make an outgoing call (commonly called glare). 

Interfaces for collecting data from the PBX: 

  • File – A file with call records of the PBX is generated by the PBX. 
  • Network port (listen mode) – The TCP or UDP port is connected by an external program. In the PBX the application receives information. 
  • Network port (server mode) – PBX communicates with a buffer or an app. 
  • Serial interface – Each call record has been historically printed on a serial printer. A software program connects to this port with contemporary systems through a serial cable. 

A PBX or other telecom system data record providing statistics for phone calls is typically referred to as a Call Detail Record or Messaging Detail Record for stations (SMDR). 

Hosted PBX systems 

Virtual PBX systems or Hosted PBX systems provide PBX functions as a service, available over the PSTN or the Internet. PBXs hosted by the phone company or service provider are generally supplied by equipment situated at an exchange facility or data centers of the provider. This implies that the client needs no PBX equipment to be purchased or installed. In general, the service is rented and the supplier may utilize the same switching equipment in some configurations to serve numerous PBX-hosted clients. 

Compared with many local systems at that time, the earliest hosting PBX services were extensive in functionality. Certain PBX services, including as follow-up, debuted via a hosting service in advance of becoming accessible on PBX hardware. Updates and new offers have been moved in both directions since launch. PBX services that comprise minimum capability sets to sophisticated combinations may be received. 

Apart from the functionality of PBX systems on the premises, PBX hosting: 

  • Allows a single number, even when it is scattered geographically, to be shown to the whole organization. An enterprise might also opt to have no premises with employees connecting home with the same functions as a PBX user. 
  • Allows multimodal access when network connectivity is provided by workers using a variety of business telephone systems, including POTS, ISDN, cellphone, and VOIP. This enables one ring extension in several places (either concurrently or sequentially). 
  • Allows scalability so that if additional workers are hired a larger system is not needed and resources will not be squandered if the number of employees decreases. 
  • Removes enterprises’ need to manage or pay for on-site maintenance for hardware. 
  • Supports integration with bespoke toll schemes (thus enabling the intra-corporate calls to be called at lower rates even from private premises), and integrated accounting and invoicing (where calls to the company on a private line but on behalf of the company are centrally billed). 

Hosted PBX providers 

In combination with an increase in Cloud Communication, the continued migration of most major communications carriers to IP networks has led to a substantial increase in the deployment of host PBX systems. 

Mobile PBX 

A mobile PBX or mobile business telephone system is a host PBX service that enhances PBX fixed-line capabilities by providing them as extensions for mobile devices such as mobile phones, smartphones, and PDA phones. Fixed-line phones may also be included in mobile PBX systems. Mobile PBX systems vary from other hosted PBX systems, where the data or calls are simply sent to mobile phones by enabling a mobile phone itself to operate PBX telephone operations without needing first to call into the system via buttons, keys, and other input devices. 

A mobile PBX can leverage the features available on a smartphone to operate the PBX operations using bespoke applications. 

Furthermore, a mobile PBX may establish extension IDs for each device, allowing the use of the extension shortcut to contact other phones in the PBX instead of a PSTN number. 


An Internet protocol IP PBX handles voice signals that provide benefits to computer telephony integration (CTI). An IP-PBX may exist as physical hardware or can perform practically the tasks of the conventional PBX or the key business telephone system as a software system for call-routing purposes. Also dubbed “Soft PBX” is the virtual version. 

Roosho is a Telecommunication engineer with more than 10 years of experience in VoIP and Unified Communications. His expertise has helped him complete more than 100 projects for Feds, Public Universities, Large Group of Companies in his 10 years of experience, and he is still growing with the industry. He loves to share his ideas about his experience and expertise with the world. That’s why VoIP Bible has made him the lead technical content writer of VoIP Bible.


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