When a voicemail message is left the message light is not coming on at the desk set (DKT 2020SD phones). Problem is on all 4 phones. The message is there but no indication. I can turn on the message light manually from another phone. The message light feature was working but stopped.

Sounds like MWI not being sent by the VM...Identifications code?

The VM system that sets station Message Waiting (MW) LEDs by dialing 6 3 + station number.
This insures that the message LED remains flashing until all VM messages have been retrieved,
at which time the VM machine should cancel the MW LED by dialing 6 4 + station number.

Perhaps...it's been years.

Thanks Professor. Its been a long while since I have worked with this also. I'm going to double check the message parameters in VM program before replacing stuff. Or I might get the customer to upgrade. They know to check their messages when they come in even if the light isn't on. Appears to have been this way for a couple of month. mailboxes filled up, age of messages, etc.