PURPOSE: To ensure the instantaneous connection between the central office incoming call and the extension telephone of the called subscriber without giving the release once to the connection to the extension telephone of the called subscriber who answered to the paging call nor dialing the extension number.

CONSTITUTION: In case the incoming arrives at central office trunk BWT from the main wire and the called subscriber is absent, the operator of central office attendant board ATT retains the state of extension mutual trunk IOT which passed through channel switch SW in the central office attendant board state memory of memory device MEM and via the CPU. Under these conditions, when the called subscriber desires the talking from extension telephone STA, the CPU receives this operation via attendant board control unit ATTCONT, confirms that the extension mutual busy state of the attendant board memory plus the call-back memory are being retained, controls switch control unit SWCONT, draws the information of central office trunk BWT and telephone STA, switches SW to connections (3) and (4), and then connecting telephone STA to board ATT and trunk BWT via line circuit LC.