The DND key was NOT made for ACD usage and should not be used for that (what you get is that the telephone gets called behind the scenes and andwers DND, so some time is wasted before trying a different rep).
Asterisk has a mechanism called PAUSE that is used to implement the DND in the ACD world, though if you use "real" Agent/xxx channels there is no need to set the DND while on call because Asterisk will do that for you (and you will use PAUSEs to signal general breaks from the ACD world). QM adds to that quite a powerful mechanism of pause tracking as well, so that the supervisor can see why people paused and what they did.
