Abstract: A method of automatically detecting fraudulent telephone calls in telephone systems having a plurality of trunks served by automatic message accounting equipment. The trunks are normally scanned to detect changes in supervisory states for automatic message accounting purposes. When predetermined supervisory state sequences are detected at any one of the trunks, a special test control signal is generated and, responsive to the appearance of the special test control signal, the transmission path of the suspected trunk is seized to monitor whatever call signaling information may appear thereon during an interval when such signaling would not appear on a normal call to verify the existence of the fraudulent usage.