then a command transmit section 307 controls the time switch circuit 51 to disconnect the data communication channel between the ta 41 and the isdn 23 and transmits a command disconnect to the isdn t point interface unit 9 and a command release to the isdn s point interface unit 19 thereafter a command release is returned from the isdn t point interface unit 9 and a command release complete is returned from the isdn s point interface unit 19 in response to both the returned commands a command transmit section 309 controls the time switch circuit 51 to disconnect the data communication channel between the data terminal 37 and the ta 41 and transmits a command release complete to the isdn t point interface unit 9 and a command disconnect to the data interface units 35 and 29 connected to the data terminal 37 and the ta 41 respectively fig1 shows the partial configuration of the microcomputer 15 which relates to the control at the called party number signal reception from the pstn 25 further the same reference numerals have been retained for similar elements shown in fig1 the above configuration is formed on the assumption that the key telephone system receives dial in service from the pstn 25 fig1 shows a communication sequence prescribed by the pstn 25 at the dial.