Abstract: In a key telephone system comprising a central control equipment and a plurality of key telephones each comprising a plurality of indication lamps for representing current states of subscriber and extension lines, additional functions such as the assignment of incoming ringing, alarm sound, all call, long distance call and international call can be checked by means of the indication lamps of respective key telephones without actually operating the key telephones. The function data stored in a function data memory in the central control equipment is rearranged in accordance with a transmission format and is transmitted to the key telephones just like as the normal use. The functions preset in the key telephone system can be checked easily and promptly on respective key telephones even by a single operator.