a switching control device for transmitting calls received by a cellular phone to a fixed - network telephone enables people to receive calls transmitted to a cellular phone by way of a fixed network telephone particularly equipped with a plurality of extension lines via conversion operations . it operates with a cellular phone / fixed network telephone switching device in cooperation with an on / off device . when a cellular phone receives an incoming call , the switching device will first detect the main telephone set and its extension lines of their individual status of use ; and the on / off device is selectively actuated of its on and off states and also its vibration ring operation according to the detection . thereby a user can pick up a nearest extension line without rushing back to a main telephone set when an incoming call is transmitted to a cellular phone .