Abstract: An extension telephone system in which a main equipment and each of a plurality of key telephone sets are connected together through paired information conductors and paired control conductors. The key telephone system can perform various functions such as answering to an incoming call from a single central office line, holding of the single central office line, transferring the incoming call from the single central office line from one key telephone set to another key telephone set, keeping the central office line in secrecy, incoming call indication, line busy indication, line holding indication and individual paging with electronic sound or voice by variation and polarity reversal of a DC voltage applied between the paired control conductors by the main equipment and utilization of the information conductors for a central office line communication, internal communication and individual paging control.