A telephone system communicating through a plurality of external telephone lines includes base phones connected to respective ones of the plurality of telephone lines and a cable connected so as to allow communication among a plurality of terminals. Each base phone includes a dual-line adaptation function unit for establishing communication through the plurality of telephone lines and a single-line adaptation function unit for establishing communication through a telephone line connected to the base phone itself. Any one of the dual-line adaptation function unit and the single-line adaptation function unit is selectively enabled in response to an external operation. Therefore, the single base phone attains a function as a single-line-adapted model establishing communication through the telephone line connected to the base phone itself or a multiple-line-adapted model establishing communication through the plurality of telephone lines.