In a key telephone system in which a plurality of data terminals 37l . . . 37n such as personal computers are connected to a key service unit 1 connected to a plurality of telephone lines 25l . . . 25m, a modem 43 is pooled so as to be usable in common for the plural data terminals 37l . . . 37n. In response to a called party number signal from the telephone lines (e.g. 25l) in dial-in service, the key service unit 1 selects a single data terminal designated by an extension telephone designation signal in dial-in service from among the data terminals 37l . . . 37n, and closes a communication channel between the selected data terminal (e.g. 37l) and the modem 43, so that a status equivalent to that where the selected data terminal 37l and the modem 43 are directly connected to each other can be established. Thereafter, the key service unit 1 indicates a called party number signal reception to the modem 43, and the modem 43 transmits a call to the selected data terminal 37l. When the selected data terminal 37l responds to the call from this modem 43, the key service unit 1 closes a communication channel between the modem 43 and the telephone line 25l, so that a communication link between the selected data terminal 37l and the telephone line 25l can be established via the modem 43. During the process until this communication link can be established, since the selected data terminal 37l and the modem 43 are in the status equivalent to direct coupling, the key service unit 1 is not concerned in the communications between the selected data terminal 37l and thus modem 43, thus reducing the operational load applied to the key service unit 1.