You have a MWI key stuck in the ON position.
Turn on the mailbox for that extension of the MWI, so you can leave a message.
Leave a message, so you have something to delete.
Delete the message, the voicemail sends a MWI OFF to the phone system.
Verify that the feature MWI is off, then delete the MWI key.
All of this is done with the extension number assigned to the MWI, not the extension it is programmed on.
I assume you are deleting a 'secondary' MWI key.