if you're dialing in the US, last time i checked the vicidial system categorized all as NA without regard for the coding (or lack thereof) regarding DC vs no answer.
i have a script that will check for zero second na (which indicates the provider didn't even attempt to call, translates to DC), but i have recent clients who have re-called those and found a fairly large percentage are real numbers ... if called through the same provider even.
so you may need to experiment with this. a useful tool is the carrier log.
