PURPOSE:To improve the serviceability to a trunk line by executing a display on a trunk line display board, when an incoming call number of an unanswer of a trunk line B board exceeds a critical value, and setting the displayed call as an incoming call of an unanswer of the trunk line B board, when said number become below the critical value. CONSTITUTION:A trunk line trunk 30 detects a transmission from a trunk line, a scanner 80 scans periodically the trunk line trunk 30, and when an incoming is detected in the trunk line trunk 30, it is informed to a central controller 70. Then controller 70 connects an incoming call to a queue of a trunk line B board 40, when a B board waiting call number shown by a counter 110 in a main storage device 100 is equal to a critical value 120 in the main storage 100, or smaller than this value, adds '1' to the B board waiting call number of the counter 110, and executes an incoming display to the B board 40 through a signal distributing device 60. On the other hand, when the B board waiting call number shown by the counter 110 exceeds the critical value 120, the incoming call is connected to a queue of a display board 50 and the incoming display is executed on the display board 50.
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