To connect plural telephone sets through one telephone line and to independently use all the telephone sets by arranging extension telephone sets with an automatic on-line control system as a main device and similarly turning the other telephone sets connected to the same telephone line into on-line state, when one extension telephone set is turned into on-line state.

Extension telephone sets 1-6 are connected with a single telephone line so as to be mutually related. An automatic on-line control system device attached to the extension telephone sets 1-6 searches the condition of the telephone line with an on-line searcher, generates an on-line control signal, works an on-line control circuit with this signal and turns the extension telephone sets into on-line state. After one of extension telephone sets 1-6 connected to the telephone line are turned into on-line state the other extension telephone sets 1-6 connected by the same telephone line are turned into on-line state by the automatic on-line control system device as well.