I completely agree! I thought that there used to be a way to remove the DND softkey on the VVX500 phones, but that no longer seems to be the case. This is incredibly annoying since turning on DND using the desk phone is not reflected anywhere else except for that specific phone. Thus, none of the other agents have any idea that it was accidentally enabled.
It may all vary on what style / brand phone you're using. My cisco 508g hardly has an indicator, the DND tab changes from light to dark and my cisco 525g actually displays a black block with Do Not Disturb inside of it. maybe if DND is pressed the phones (users) line keys turn red like they would if the unit was in use? or blink or some type of better visual. But yes an admin should be able to check if a phone is in DND remotely, maybe DND should be a feature that should able to be selected to be turned off in templates / roles.