A signal detecting system using telephone lines for signal transmission includes at least one detecting device, at least one alarm device, and at least one telephone line. Each detecting device includes an RF receiver, a filter, a signal amplifier, a direct converting receiver, and a level converter. Each alarm device includes a controller and a warning device. By means of the signal detecting system, abnormal signals (such as signals from a pinhole camera) around a detecting area can be detected and transmitted through telephone lines, and thus activate a warning device.