PURPOSE: To implement extension talking and memo registration or the like while not disconnecting the connection of an external line and inhibiting the transmission of a voice signal or the like to an external line during talking with the external line by a slave set.

CONSTITUTION: Each of a master set A and a slave set B is provided with a transmission mute section 17 muting a transmission signal and a mode setting section 9a to set various modes such as an extension call mode and a memo registration mode or the like. Moreover, when the mode is set to the memo registration mode during external line talking, the transmission mute section 17 is operated to mute the transmission voice ad a registration data from the slave set is stored in a storage section 4 of the master set A and when the mode is set to the extension call mode, the transmission mute section 17 is activated to mute the transmission voice and to implement the extension call by a control section 10.