Once they get that DV letter they cannot call you anymore until they validate it and they cannot send any collection letters.....If they call answer only one time, If they try to collect, tell them you sent a DV letter and tell them the date they got it....if they play dumb and say they didn't get it, hang up....count that as 1 violation and any calls after that, document them, take pics of the caller ID, if they leave a message save it and record it......
Once the build up enough violations, go to NACA.net and get a consumer attorney and talk to them as you now may be able to file a lawsuit against them.....you can sue for up to $1,000 in damages plus actual damages, such as emotional distress, pain and suffering, harrasment....if you get them on that, that is where the big money comes in.....
Remember though....you do not sue for each violation, meaning it is not $1,000 per violation,,,,if they have 50 violations, it is up to $1,000 in damages and then you can put actual damages......the more violations you have in the lawsuit the better, it shows they do not comply and most likely can't use the bona fide error defense.....If they only called once and you sued them, most likely you would lose.