Fuente: WIPO "rice" 1,187,487. Automatic exchange systems. C. F. RICE and C. KIMTANTAS. 16 Sept., 1967, No. 42309/67. Heading H4K. A subscriber is provided with two (or more) exchange lines L1, L2 to which his set may be connected alternatively by a switch 34, and the lines may be coupled together by a transformer 17, the windings of which are normally ineffective on account of series-connected silicon controlled rectifier switches 13, 25, the latter being bi-directional and being made conductive by the actuation of push-buttons 19, 30 to apply control voltages to trigger electrodes 20, 31. Thus, a call may be set up or received over either line, an incoming call on one line may be transferred over the other one, or a conference call may be established. Keys 11a, 23a provide for release if necessary. The circuitry may be incorporated in a box located near the set, and the control key or buttons may be on the set itself. Operation is independent of line polarities.