PURPOSE: To prevent effectively and economically malicious callings with only the system used for normal call connection by arranging the constitution with which only those who know the special key number can use the telephone.

CONSTITUTION: In case where a subscriber SUB1 of local station LS desires to prevent malicious calls, he shall write and register the special key number with the special area of subscriber's memory M by means of the central processor system CC of LS. Then, when an incoming call from another subscriber SUB2 reaches LS, CC gains access to special area M of the dialed subscriber number, distinguishing (a) that he is SUB1 whose key number has already been registered and, then, immediately transmits (b) a key number request signal (dial tone) to SUB2 and when SUB2 had dialed the specified key number and it is assured to be the correct number CC, in-trunk ICT is connected to ring back tone RBT and SUB1 is connected to ringing trunk RGT, thus transmitting ring back tone and ringing signal (c).