the presence of such signals is detected each time the following detection condition is satisfied , namely : that , for the duration of a predetermined &# 34 ; prior activity &# 34 ; period the amplitude of the signal on the telephone channel exceeds a threshold level while successive maximum values taken at short time intervals remain fairly constant in level . apparatus for performing the method comprises a threshold level comparator and a difference comparator which control a detector followed by a holdover circuit . the invention is particularly applicable to inhibiting echo suppressor circuits during telephone signalling or data transfer by telephone line .