A key telephone system in general consists of a main device, telephone sets, lines connecting the main device with the telephone sets and an additional unit for operating the system more efficiently. There're also cordless key telephone systems developed to replace wired key telephone systems to reduce the amount of cables and wires laid on office floors. The system usually includes a main controller which is coupled to the public or private switched telephone network via exchange lines to receive incoming calls and originate outgoing calls. A cordless station is provided with a field intensity detector for alerting the user when the field intensity drops below an acceptable level. A conventional key telephone system is comprised generally of a telephone including selection buttons which emit a light to thereby inform a user of a public switched line through which a call is made, a key telephone unit which controls a telephone in transmitting a call and receiving a call, and a public switched line connected between the key telephone unit and a central telephone exchange. When an incoming call is directed to such a system, a lamp is lit on each telephone set to indicate an incoming call. The button associated with the ringing line can be pushed on any telephone set so that any party can respond to the incoming call and inquire as to the identity of the desired called party. The party initially answering the key telephone set then communicates, either verbally or by way of intercom or public address broadcast facilities, the identity of the called party and the line number associated with the incoming call. In a key telephone system, a key telephone set is assigned a main extension number and a pseudo extension number which is the number assigned to the main extension without a physical accommodation position and a key telephone set. A key telephone system is generally designed to include further two lines through which the above-mentioned signal for turning a selection button on is transmitted or received, as well as two telephone lines necessary for making communication therethrough. That is, a key telephone system is usually designed to include four telephone lines. The switching function of line selection is mechanically provided and distributed among the key telephone sets. Any features in addition to plain ordinary telephone service (POTS) must be added on a per line basis. The primary advantage of these systems is economy with small size. However, if such a system is required to expand along with the organization it serves, over a time it eventually becomes more expensive on a per line and feature basis than a private branch exchange would be.

A digital key telephone system performs data transfer on a data channel based on ping-pong transmission between the key telephone unit and a key telephone set. The ping-pong transmission is a system for transmitting data by generally using the key telephone unit as a master device and using the key telephone set as a slave device. The key telephone set extracts data transmitted via the data channel from the key telephone unit by use of a ping-pong transmitting section and then reads the extracted data by use of a CPU. A digital key telephone system comprises a main unit connected to a public network or a private branch exchange and a plurality of digital key telephones connected to the extension lines of the main unit. The digital key telephone has a plurality of push button type keys. When these keys are depressed, calling the public network or the private branch exchange, answering a call, and speech communication between digital key telephones are allowed. The main unit processes not only a control signal but also a speech communication signal as a digital signal and executes calling processing/call reception processing of each digital key telephone by a stored program control scheme and switching processing by a time switch of the digital speech communication signal. A key telephone system commonly employs a technique for reducing the communication cost, such as a least cost route (LCR). In order to perform the LCR function, a database should be built upon a close examination of traffic intensity and the communication networks surrounding the operating site. Out of the LCR-related databases, a DCT (digit comparison table) database is most closely related to the search time of the database. The DCT database has hundreds of numerical strings programmed therein. The key telephone system employs a table search method consisting of an insertion sort algorithm and the binary search algorithm, to reduce the search time. The insertion sort algorithm is used to build the database, and is the most simple one of the known sort algorithms.

In recent years, digital key telephone apparatuses with an ACD (automatic call distributor) function have become popular. In the ACD function, a plurality of extension terminals constitute a single ACD group, and incoming calls from a network corresponding to the ACD group are equally distributed to the plurality of extension terminals in the ACD group. With this ACD function, a digital key telephone apparatus can prevent incoming calls from being concentrated on a specific extension terminal, and can efficiently process incoming calls. In a key telephone system, the communication between main and terminal devices is performed in such a manner that a key code is transmitted from the terminal device to the main device, or a control instruction code is transmitted from the main device to the terminal device. A computer communication interface is included in the terminal device to interface audio or data communication between the main device and a transceiver of the terminal device, and also interfaces data communication between the main device and a computer connected to the system through a separate RS232C line. By doing so, computer communication can be carried out through the main device connected to an external public telephone network. When the digital key telephone system is connected to such a communication network, the main unit of the called digital key telephone system detects the first bell signal and then a modem signal. When calling line identification information is detected from the modem signal, the calling line identification information is transferred from the main unit to the called key telephone and displayed on the liquid crystal display device (LCD) of the key telephone. The user of the key telephone can recognize the calling line before off-hook by checking the calling line identification information displayed on the LCD display.