There are varioud reasons why RAD's go into DND. the man reason is that the timers are wrong in the COS. It is always good to have DND enabled, therefore, you will know that some programming is not right.
If you disable, do not know what will really happen, I would of thought that instead of DND that the it will do into another kind of state, that again will not play any messages.
Overall, if the setup is correct the RAD's should not go into DND. Make sure of the following:
When using 3300 Embeded RAD's ensure that you put them into a Hunt Group with a phase timer of 1 or more and use the Hunt Group number instead of the RAD number.
Ensure that the timings are right in each COS for each RAD.
I hope this information helps