A business telephone system differs from an installation of several telephones with multiple central office (CO) lines in that the CO lines used are directly controllable in key telephone systems from multiple telephone stations, and that such a system often provides additional features related to call handling. Business telephone systems are often broadly classified into key telephone systems, and private branch exchanges, but many hybrid systems exist.
A key telephone system was originally distinguished from a private branch exchange (PBX) in that it did not require an operator or attendant at the switchboard to establish connections between the central office trunks and stations, or between stations. Technologically, private branch exchanges share lineage with central office telephone systems, and in larger or more complex systems, may rival a central office system in capacity and features. With a key telephone system, a station user could control the connections directly using line buttons, which indicated the status of lines with built-in lamps.
System components PBX often includes:
- Cabinets, closets, vaults and other housings.
- Console or switchboard allows the operator to control incoming calls.
- Interconnecting wires and cables.
- Logic cards, switching and control cards, power cards and related devices that facilitate PBX operation.
- Microcontroller or microcomputer for arbitrary data processing, control and logic.
- Outside telco trunks that deliver signals to (and carry them from) the PBX.
- Stations or telephone sets, sometimes called lines.
- The PBX's internal switching network.
- Uninterruptible power supply (UPS) consisting of sensors, power switches and batteries.
Home and small-business usage
Historically, the expense of full-fledged PBX systems has put them out of reach of small businesses and individuals. However, since the 1990s many small, consumer-grade and consumer-size PBXs have become available. These systems are not comparable in size, robustness or flexibility to commercial-grade PBXs, but still provide many features.
The first consumer PBX systems used analog (POTS) telephone lines, typically supporting four private analog and one public analog line. They were the size of a small cigar box. In Europe these systems for analog phones were followed by consumer-grade PBXs for ISDN. Using small PBXs for ISDN is a logical step, since the ISDN basic rate interface provides two logical phone lines (via two ISDN B channels) which can be used in parallel. With the adoption of VoIP by consumers, consumer VoIP PBXs have appeared, with PBX functions becoming simple additional software features of consumer-grade routers and switches .Additionally, many telecommunications providers now offer hosted PBX systems where the provider actually hosts the PBX and the phone handsets are connected to it through an internet connection.
Open source projects have provided PBX-style features since the 1990s. These projects provide flexibility, features, and programmability.
Launched in 1997, IP-PBX took business communication to the next level. IP-PBX is a PBX phone system that uses Internet Protocol (IP) data networks to manage the routing and switching of calls as well as to handle messaging. Virtual hosting resulted in significant cost efficiencies. Apart from offering advanced features (like voicemail), the system could also use a VoIP gateway to connect to traditional PSTN lines. This gave the user the option of continuing with the same carrier. Over time, improvements in the user interface and call quality, along with cost benefits, added to the popularity of IP-PBX among small businesses.