A business telephone system is any of a range of a multiline telephone systems typically used in business environments, encompassing systems ranging from small key systems to large scale private branch.
A business telephone system differs from simply using a telephone with multiple lines in that the lines used are accessible from multiple telephones, or "stations" in the system, and that such a system often provides additional features related to call handling. Business telephone systems are often broadly classified into "key systems", "hybrid systems", and "private branch exchanges".
A key system was originally distinguished from a private branch exchange (PBX) in that it allowed the station user to see and control the calls directly, manually, using lighted line buttons, while a private branch exchange operated in a manner similar to the public telephone system, in that the calls were routed to the correct destination by being dialed directly.
Technologically, private branch exchanges share lineage with central office telephone systems, and in larger or more complex systems, may rival a central office in capacity and features. Hybrid Office Phone Systems
Into the 21st century, the distinction between key systems and PBX has become increasingly confusing. Early electronic key systems used dedicated handsets which displayed and allowed access to all connected PSTN lines and stations.
Nortel T Series Key System Telephone
The modern key system now supports SIP, ISDN, analog handsets (in addition to its own proprietary handsets - usually digital) as well as a raft of features more traditionally found on larger PBX systems. Their support for both analog and digital signalling, and of some PBX functionality gives rise to the "Hybrid" designation.
A hybrid system typically has some call appearance buttons that directly correspond to individual lines and/or stations, but may also support directly dialing to extensions or outside lines without selecting a line appearance.
The modern key system is usually fully digital (although analog variants persist) and some systems embrace VOIP. Indeed, key systems now can be considered to have left their humble roots and become small PBXes. Effectively, the aspects that distinguish a PBX from a hybrid key system are the amount, scope and complexity of the features and facilities offered.
Hybrid systems are a common tool in the financial services industry used on trading floors. These advanced hybrid key systems generally only require attached PBXs for interaction with backroom staff and voicemail. These systems commonly have their front end units referred to as Turrets and are notable for their presentation of hoot-n-holler circuits. Multiple Hoots are presented to multiple users over multiplexed speakers to multiple locations.Private Branch Exchange
An Avaya G3si PBX with front cover removed (view from the top).
A private branch exchange (PBX) is a telephone exchange that serves a particular business or office, as opposed to one that a common carrier or telephone company operates for many businesses or for the general public.
PBXs are also referred to as:
PABX - private automatic branch exchange
EPABX - electronic private automatic branch exchange
PBXs make connections among the internal telephones of a private organization—usually a business—and also connect them to the public switched telephone network (PSTN) via trunk lines. Because they incorporate telephones, fax machines, modems, and more, the general term "extension" is used to refer to any end point on the branch.
PBXs are differentiated from "key systems" in that users of key systems manually select their own outgoing lines, while PBXs select the outgoing line automatically. Hybrid systems combine features of both.
Initially, the primary advantage of PBXs was cost savings on internal phone calls: handling the circuit switching locally reduced charges for local phone service. As PBXs gained popularity, they started offering services that were not available in the operator network, such as hunt groups, call forwarding, and extension dialing.Latest Trends in IP Business Phone Systems
One of the latest trends in PBX development is the VoIP PBX, also known as an IP-PBX or IPBX, which uses the Internet Protocol to carry calls. Most modern PBXs support VoIP. ISDN PBX systems also replaced some traditional PBXs in the 1990s, as ISDN offers features such as conference calling, call forwarding, and programmable caller ID. However, recent open source projects combined with cheap modern hardware are sharply reducing the cost of PBX ownership.
For some users, the private branch exchange has gone full circle as a term. Originally having started as an organization's manual switchboard or attendant console operated by a telephone operator or just simply the operator, they have evolved into VoIP centres that are hosted by the operators or even hardware manufacturers.
These modern IP Centrex systems offer essentially the same service, but they have moved so far from the original concept of the PBX that the term hardly applies at all.
Even though VoIP gets a great deal of press, the old circuit switched network is alive and well, and the already bought PBX's are very competitive in services with modern IP Centrexes.
Currently, there are four distinct scenarios in use:
PBX (Private and Circuit Switched)
Hosted/Virtual PBX (Hosted and Circuit Switched) or traditional Centrex
IP PBX (Private and Packet Switched)
IP Centrex or Hosted/Virtual IP (Hosted and Packet Switched)
Since in reality people want to call from the IP side to the circuit switched PSTN (SS7/ISUP), the hosted solutions usually have to maneuver in both realms in one way or another. The distinctions are seldom visible to the end user.Source :