PURPOSE: To exactly select any trunk line by judging whether the number of dialing is suited to the contents of service set corresponding to trunk lines or not, automatically selecting the trunk line corresponding to the result, and transmitting the number as a dial signal later.

CONSTITUTION: When operation data are transmitted from a telephone set 51 to a main device 1, a CPU 14 is shifted to a service setting mode and sets a trunk line number in the prescribed area of a RAM 16. When the operation data of a dial key 57B to be operated at the telephone set 51, the CPU 14 respectively judges whether the data show the discount trunk line of a long-distance call or the data show the discount trunk line of a long-distance call or not. In this case, when the operation data show the discount trunk line of the local call, a local call originating mode corresponding 5o the trunk line number set to the RAM 16 is set. When those data show the discount trunk line of the long-distance call, a long-distance call originating mode corresponding to the set trunk line number is set.