Message Waiting Lamp. The 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. Note: The LM-1A is not compatible with any AT&T phone system. Applications Message waiting indication on analog lines supplying standard message waiting voltages Ring indicator in offices Features Attractive styling Easy retrofit mounting Works with nearly all message waiting voltages One year warranty Made in the USA.