DNC is useless. I have caller ID and call blocking. I have blocked this number and 29 other numbers, 30 the limit for my phone system. What I do is roll off the number that has been blocked the longest (on the bottom of my list), adding new numbers to the top of my list. It seems to work fine that way as those who have been blocked the longest finally get tired of being blocked. When I get a replacement phone, I will look for one with a higher capacity for blocking calls.
I could block more calls for a monthly fee of about $8 by my service provider, but refuse to spend nearly $100 per year on that kind of service.