SUBSCRIBER LINE EXCHANGE

PROBLEM TO BE SOLVED: To know if the same person repeatedly makes a telephone call by providing a called side subscriber line exchange with an enciphering device, which enciphers the full digits or a partial digit of a calling telephone number that is transmitted to a called side telephone set. SOLUTION: A called side subscriber line exchange 3 is connected to a called side telephone set 1 through a subscriber line 2, and is also connected to a calling side subscriber line exchange through a common signal line 4. A calling telephone number that reaches the exchange 3 goes through a common line signal equipment CSE 39 and a call control processor 38, and enters a cipher converting device 52. The device 52 converts the calling telephone number into a enciphered character, based on a character conversion table stored in a cipher storage device 51. The converted caller's telephone number is sent to a transmitting device 34, converted into a signal format which the calling side telephone set 1 can receive and transmitted to the set 1 through a subscriber line 31. The converted caller's telephone number is displayed on a display of the set 1.