back tone signal portions .Iaddend.each .[.applied for.]. .Iadd.having .Iaddend.a preselected duration .[.and.]. with a preselected interval between successive ringback .[.signals,.]. .Iadd.tone signal portions, .Iaddend.and including timing means for timing the application of the communication signals such that the latter are applied during the intervals between successive ringback .[.signals.].