The Viking LM-1A universal message waiting lamp retrofit kit is compatible with any telephone system with standard DC message waiting voltages ranging from 90VDC to 120 VDC. The LM-1A is fully modular for quick plug-in installation by non-telephone personnel. The unit's LED will blink at a steady rate when the message waiting voltage is detected. The LM-1A will also flicker when the phone rings, helping to reduce noise in quiet areas by providing a convenient visual indication for ringing or message waiting.