Affordable test phone with extensive features. The Compact DSP (Digital Service Protection) has been developed to ensure safe operation on lines where digital signals, network, remote power, or fault voltages may occur, caused by defective or swapped cables. Once you connect the device to a line with a voltage greater than 75 V/DC (analog systems are powered with voltages from 24 V to 66 V), an alarm signal (digital voltage alarm) will sound. This indicates that it is not an analog line, but a digital one. The built-in monitor function allows you to determine if digital signals are superimposed on active lines without having to disconnect the line. The high voltage lock prevents the interconnection of lines with a DC supply of more than 110 V in speech mode. DSP not only protects ISDN and double line services from interference but also protects the testing device from damage that could result from consuming high power levels in speech mode. All DSP functions work even with a dead battery.