message reg command for requesting setting of terminal registration to the ats 122 in step 215 a call state is changed to the state s 6 for waiting for the reg complete message from the ats 122 and a fifth sensing timer of the state s 6 is operated the acs 123 measures a time after the state is changed to s 6 by the fifth sensing timer and a function fns 6 timerexpirey is operated when the acs 123 does not receive the reg complete message before a predetermined time of the fifth sensing timer expires the ats 122 finishes the setting of the terminal registration and transmits the reg complete message to the acs 123 in the state s 6 when receiving the reg complete message from the ats 122 in step 216 the acs 123 calls a function fnreg complete when the function fnreg complete is operated the acs 123 forms a registration request message regreq for requesting the setting of the terminal registration to transmit it to the par 130 in step 217 a call state is changed to the state s 7 for waiting for the regrsp message from the par 130 in step 236 and a sixth sensing timer of the state s 7 is operated the acs 123 measures a time after the state is changed to s 7 by the sixth sensing timer and performs a function fns 7 timerexpirey when the acs 123 does not receive the regrsp message before a predetermined time of the sixth sensing timer expires in the state s 7 when receiving the regrsp message from the par 130 in step 218 the acs 123.