A supervisory circuit in a telephone switching system includes two current sensing transistors, each adapted to detect current flow in alternate directions through a subscriber''s loop serving one or more stations. The conductive or nonconductive state of the transistors in combination sequentially defines supervisory states which represent on- and off-hook indications, dialed digits, as well as party identification. The party identification is utilized to ascertain which of the plural stations served by a party line is presently instituting a call.