section 37 to the time division switch 8 where it is conveyed in the direction of the arrow p before it is supplied to the second input terminal of the hands free trunk 9 which terminal is coordinate with the first input terminal then the output from the hands free trunk 9 is transmitted in the direction of the arrow q and enters the time division switch 8 again where it is conveyed in the direction of the arrow r and is transmitted to the telephone line 35 via the analog outside line interface section 36 in this way a one way conversation can be realized thereby preventing the generation of a howling noise which would otherwise be caused when transferring an outside call to an outside line by a reflection of the transmission signal as a result of a mismatching of the balancing circuit due to the conversion from two wire to four wire system the present invention is not limited to the above embodiments various modifications are possible without departing from the spirit and scope of the invention for example while in the above embodiments signal input to the hands free circuit is effected via two highways it is also possible for two reception signals to be input to a single highway by being superposed on two time slot signals thereby simplifying the circuit construction further while in the above embodiments the hands free circuit uses analog signals it is also possible to adopt digital processing comparing the levels of two digital signals and selecting the one having a higher level.