Circuit preventing listening-in at telephone exchange - detects data signals or code signal on voice line and may be permanently or intermittently connected to line

The telephone exchange prevents listening-in and crossed lines when data is being transmitted via voice lines. A circuit is intermittently or continuously provided in the connection path to detect the presence of data signals or of a code signal on the voice line. This circuit produces a signal that prevents the connection being accessed by other parties. The detector can be switched to any one of a number of connections and consists of an AF receiver tuned to the AF signal that marks the quiescent transmission path.