A method and apparatus for displaying duration of a call that is made over a central office line. The keyphone system having a plurality of keyphone subsets with displaying devices, for carrying out the invention, includes the steps of: notifying call-start by activating a call-duration start timer, if the call over the central office line is started, or, if not, checking whether or not the call is ended; checking whether or not the call-duration display is held, if the call is ended in the above step; checking sequentially whether or not a signal for activating call-duration start timer or call-duration increment timer are activated, if the call is not ended and a time key is not pressed; checking whether or not the central office line is held, if the signal for activating the call-duration start timer or call-duration increment timer is activated, and, if not held, sending call-duration data to a calling keyphone; and clearing a display of the call-duration and displaying a current time, if the time key is pressed and call-duration display is held in the checking step of the signal for activating call-duration start timer or call-duration increment timer.