PURPOSE: To improve the efficiency of jobs by eliminating operations excepting for the speaking of an operator and waiting time by automatically originating a call to a trunk line and connecting the terminal of an idle, line and a completely connected trunk line on the opposite side when the opposite side is completely connected.

CONSTITUTION: When the data of a trunk line number are transmitted from an input/ output device 59, a central control part 56 controls respective trunk line control parts 41, detects any idle trunk line 47 and catches this trunk line 47. Afterwards, the central control part 56 transmits a trunk line dial signal to a dial transmitting part 42, and the dial transmitting part 42 transmits the trunk line dial signal through a channel control part 43 to the trunk line 47. Afterwards, a response detecting part 45 inside the trunk line control part 41 detects the response of a telephone set on the opposite side, the central control part 56, that receives this detecting signal, outputs a command to an extension selecting part 58 and the extension selecting part 58 selects any extension 54 to be connected with the responding trunk line 47 out of the extensions 54. Then, the central control part 56 outputs the command of bell transmission to a bell transmitting part 50 inside an extension control part 49 connected to the extension 54, and speaking is started.