The LM-1A relies upon a DC voltage to be applied to the phone line to detect a "Message Waiting" condition from the phone system. ( 68 VDC minimum).
Some phone systems/providers utilize "stutter dial tone" (one hears a "broken" dial tone when picking up the phone), instead of the DC voltage referenced above.
The LM-1A is not designed to detect “stutter dial tone” but will flash on incoming calls.
The VR-1B model can properly detect “stutter dial tone” and therefore provide message notification in this instance.