tone trunk tnt is inserted into the time slot ts i 1 at point d further the signal inserting circuit i 1 arranged between the switching network sw and the receiving section of a line circuit portion inserts sd data into the time slots ts 0 through ts i therefore at point b the contents of the frame of data is as is illustrated in fig7 b and thereby dial tone signal data is transmitted to the subscriber a and sd data is transmitted to the receiving section of each of the subscriber circuits in the above mentioned switching operation the writing of data into the speech path memory and the reading out of data from the speech path memory are effected under the control of the common control unit pu by the control memory in the switch controller sc the writing in of data transmitted from the input transmission line is effected sequentially in the order of the time slot numbers that is the data existing in each of the time slots of the transmission line is sequentially written into the addresses of the speech path memory the addresses each corresponding to a time slot for example the data of the time slots ts 0 ts 1 ts n is sequentially written into the.