Digital button telephone system and extension terminal for the same

According to this invention, a digital key telephone system connected to an analog public network NW having a function of transmitting a ringing signal including identification information of a calling line through a subscriber line (CO line), accommodating a plurality of extension lines each connected to a digital key telephone (DKT) 2 or a standard telephone (STT) 4 as an extension terminal, and having a function of switching and connecting the subscriber line to the plurality of extension lines or the extension lines to each other includes a called party storage means storing, in advance, information representing the correlation between the calling line and the extension terminals 2 and 4 as a called terminal. When a ringing signal arrives from the analog communication network NW, calling line identification information (caller ID) contained in the ringing signal is detected by a calling line identification information interface unit (RCIU) 12. A control unit (RCTU) 16 determines the called extension terminal on the basis of the detected caller ID and the information stored in the storage means, so the extension terminal receives the call from the digital key telephone interface unit (RDKU) 13 or a standard telephone interface unit (RSTU) 15.

TECHNICAL FIELD
 The present invention relates to a digital key telephone system provided in
 a company or an office.
 BACKGROUND ART
 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.
 As one of analog communication networks provided in U.S.A. recently, there
 is a communication network for providing a service called, e.g., "

BEST MODE OF CARRYING OUT THE INVENTION
 FIG. 91 is a view showing a signal format used in a communication network
 using a