PURPOSE: To attain privacy call function and talking and transfer between telephone set without complicated wiring by connecting a call detection circuit, an extension signal transmission/reception circuit, a changeover means switching a telephone line call and extension call and an adaptor comprising a central control means to each telephone set.

CONSTITUTION: Extension telephone sets 2, 3, 4 are connected in parallel with a telephone line 1 via an adapters 5. When a call detection circuit 17 detects an external line call signal from the line 1 and hooks off a handset 6, it is detected by a DC loop detection circuit 21 and a CPU 16 energizes a relay 11. The direct call between the extension telephone sets is implemented by an extension signal reception circuit 18 and an extension signal transmission circuit 19. In case of extension call, the CPU 16 switches relays 12,13 to the position of an impedance circuit 14. In calling other extension telephone set for transfer or extension call, a call confirming signal generating circuit 23 sends a call tone to inform a calling state to the telephone set at the caller side. If a call comes from an external line or other extension, the call signal generating circuit 24 sends the call signal.