PURPOSE: To obtain the master-slave telephone system in which the operability of internal transfer and the practicality are improved by a holding state extension calling.

CONSTITUTION: The system is provided with a holding state extension call means that holds a public line 1 in external line connection to a 1st telephone set 2 to apply extension calling to a 2nd telephone set 6 from the 1st telephone set 2 and with a transfer control means that sets an extension speech state to the 1st and 2nd telephone sets when the 2nd telephone set hooks off the call by the holding state extension call means and the st telephone set 2 is not hooked on and that connects the holding public line to the 2nd telephone set 6 when the 1st telephone set is already hooked on.