PURPOSE: To realize an economical service charge control of each key telephone set, by feeding the information on the call to a service charge controller from an electronic key telephone device.

CONSTITUTION: A transmission interface 2 provided to a main device ME of an electronic key telephone device is connected to a transmission interface 3 provided to a service charge controller 1 via a signal cable 4 consisting of data transmission lines l1∼l3. Service information from key telephone sets T1∼Tm is transmitted to the controller 1 in order of the using telephone number and the telephone number of the remote side through the line l1 of the cable 4 and via the interface 2 of the device ME. Thus the service charge is calculated. The line l2 transmits to the device ME whether or not the controller 1 can receive the data from the device ME. The line l3 is used to request again the transmission of data to the device ME if an error is contained in the data received by the controller 1 from the device ME.