A reverse power feeding PSE in electrical communication with a DPU, the PSE constituted of: a control circuitry; and a detection and classification signal circuitry, arranged to: output a detection signal; output a classification signal; and output an additional signal exhibiting a voltage below an operating range, and wherein the control circuitry is arranged to: responsive to the output first detection signal, detect a valid signature resistance; responsive to the output first classification signal, receive a classification current and determine the class of the DPU; responsive to the output additional signal, detect the absence, or presence, of an off-hook phone; responsive to the detection of the absence of an off-hook phone, and further responsive to the detection of the valid signature resistance, output power exhibiting a voltage within the operating range; and responsive to the detection of the presence of an off-hook phone, not output power.