PURPOSE: To make it clear which single telephone set is in a call state.

CONSTITUTION: When a pushbutton telephone set 2 makes an extension call to a single telephone set 3, a pushbutton telephone set interface section 6 sends a dial pushbutton telephone set number and an opposite extension dial data to a main control section 8. The main control section 8 sends a caller pushbutton telephone set number to a control section 7-3 of a single telephone set interface section 7. The control section 7-3 of the single telephone set interface section 7 accesses a memory section storing a call signal pattern of the single telephone set 3 in advance according to the inputted caller pushbutton telephone set number to read a call signal pattern and controls a ring signal changeover section 7-2 in the single telephone set interface section 7 according to the call signal pattern and sends the result to the single telephone set 3. As a result, the single telephone set 3 rings at an interruption period exclusive for the single telephone set 3.