In general, a private branch exchange (PBX) switches an incoming call from an office line or an extension line to a specified extension line in response to a call switching request. Within a corporate environment, telephone service is typically provided by a PBX switch. A PBX switch is an on-site facility, that is typically owned or leased by a company or another entity. The PBX interconnects the telephones within the facility and provides access to the public telephone system. A private branch exchange (PBX) is a circuit switch that serves a community of terminals, such as telephones, facsimile machines, and computers. A private exchange system switches a plurality of extension terminals within the same building or the same office, including a key phone system, a private branch exchange, and a private branch automatic exchange (PBAX). By interconnecting the terminals, the PBX provides an inexpensive and convenient communication network among the community of terminals. The PBX is also connected to nodes in the public switched telephone network (PSTN), thereby providing communication paths to terminals outside the network. In a basic private branch exchange configuration, the PBX is connected between the central office switch and the individual telephone stations. A PBX may be connected to a switching office of the public telephone network, and multiple PBXs may be tied together to handle a large number of telephone lines. The PBX concentrates signals on the trunk lines for subsequent transmission to a network of private stations. The PBX equipment provides for switching calls between any two extensions served by the exchange or between any extension and the national telephone system via a trunk or trunks to a central office. The PBX equipment provides switching functions which permit subscribers, under certain limitations, in an office environment to gain access as required to trunk lines, including WATS (wide area telephone service), DDD (direct distance dialing) and the like, for enabling connections of telephone calls. The telephone terminals connected to the PBX operate in one of three different modes. In a regular user mode, the user is able to receive incoming calls directed to a particular directory number and make outgoing calls. In an operator mode, the user is able to answer and route calls directed to the PBX to a particular one of the telephone terminals. In an automatic call distribution (ACD) agent mode, the user can answer calls which are directed to one or multiple subgroups of the PBX. A PBX system may include a user interface which is usually similar to a user interface of a computer system. The processing unit receives inputs from the user through a keyboard or mouse and displays outputs on a visual display. By inputting various commands, the user can cause the PBX to perform various requested operations.

A PBX phone system generally includes telephone trunk lines, a computer used for exchanging and managing calls inside and outside the PBX, a line network in the PBX, and a console or switchboard for an operator. In a PBX system, a number of lines are provided between a group of local users and a switch in the PBX system. Through these lines, the PBX system provides a certain number of accesses to a remote communications exchange and also provides access for the local users to each other. The PBX interconnects the terminals using voice and data lines which are connected to a digital switch via a voice or data interface card. The PBX is connected to the PSTN by a set of trunk lines which are connected to the digital switch via a trunk interface card. A PBX system may also include a line scanner for monitoring and detecting activity on the voice, data, and trunk lines. The PBX itself can have various configurations, but typically includes at least a controller, a switching circuit, a memory, and a tone generator. An operator station typically includes a "busy lamp field" display for displaying busy status information of all telephone extensions connected to the PBX. The display includes green and red light emitting diodes to indicate whether a particular extension is on-hook or off-hook. The display allows an operator to determine whether the telephone extension is currently engaged in a conversation when attempting to connect a caller to the telephone extension. In a computer/telephony integration a computer is connected to one of the station lines to emulate the telephone terminals with the end result of controlling most of the features offered by the PBX through the computer interface. To provide greater versatility and functionality, many modem PBXs further comprise one or more server processors which are coupled to the CP of the PBX switching unit (SWU). The server processors provide a link for communication between the CP and host computers which are external to the PBX. A PBX typically includes a private switch, usually on the enterprise's premises, connected to a common group of lines or trunk circuits running from one or more central offices to provide communications service to a number of individual phones. Within a PBX, calling parties may usually simply dial a three- or four-digit extension to connect to a called party also within the PBX. With the recent explosion of mobile unit usage by enterprise employees, many enterprises wish to extend support for three- or four-digit dialing for their employees' mobile units, a service often referred to as creating a "virtual private network."

A private branch exchange environment brings together many forms of communication including but not limited to: telephony conversations, voice or phone mail, electronic mail, and facsimile messages. The private branch exchange allows the phone to make and receive simple telephone calls, and also provides numerous additional services such as, for example, dial-by-name, voice mail, speed dial, call transfer, display of caller information, and the like. Such PBX functionality is generally achieved by the exchange of digital, vendor-specific, private signaling codes between the phone and the PBX. A modern private branch exchange system, in addition to facilitating communication among its lines and extensions must be capable of providing a number of special user services. A PBX system must be capable of generating and routing, to each line and extension, a variety of standard and non-standard informational and supervisory tonal signals, such as dial tone, audible ringing signal, to provide each user with individualized call-status information. Due to the powerful processing capability of the microprocessor, personal computers (PC) can be configured to provide an increasing variety of telecommunications applications. By incorporating at least one voice processing board into a PC, and programming the PC and voice board with appropriate software, the personal computer can provide users with voice mail, pre-recorded messages, automatic call transfer, access to different databases, interactive voice response, and speech recognition, among other telecommunications features. PBX systems used in businesses include both analog and digital PBX systems connected to two-wire analog trunks to connect their PBX to the local phone company. Telephone lines which are used for residential purposes also typically are two-wire analog lines. The widespread use of personal computers has resulted in the interconnection of such computers through modems which convert the digital signals from the computer to analog signals suitable for transmission over analog telephone lines, and for reconverting received analog signals into digital signals for processing by the computer.