PURPOSE: To discriminate a call from a terminal telephone set securely by sendig a signal, showing a class other than a specific class, from a mobile station and then by sending a signal showing the specific class in succession.

CONSTITUTION: When a terminal telephone set TELC selects a pay station circuit and sends a call signal, a signal showing a class other than a specific class and a service-system call signal fR having a frequency fSV4 or fSV2 are sent out according to the direction of a train since the telephone set TELC belongs to the pay station system as the specific class and is dedicated to the connection of a toll board, and a pay station call signal fP which shows the specific class and has the frequency fsv3 or fsv1 is sent out similarly in succession. Therefore, whe the signal transmitter- receiver SSRM detects the frequency fsv4 or fsv2 and then fsv3 or fsv1, it is discriminated a call has been made by the telephone set TELc through the pay station circuit, and consequently a connection with a toll board telephone circuit LT is made.