The biggest problem (and one that a lot of folks don't realize) with the DNC list is that third party calls don't fall under that umbrella. Say you have a credit card with a particular bank. Because you have a "business relationship" with the bank, they are still allowed to call regardless of the DNC status. I don't think anyone has an issue with that. However, if that bank has a relationship with a third party, say someone peddling carpeting,
that business (because of their ties to your bank) also can call you regardless of your numbers DNC status. And I think (but am not positive) that non-profits are exempt as well.
As far as the bill collectors ... I had that problem with my cell phone number. And the only thing that worked was to call them and firmly tell them that the jackwagon that had the number before me was apparently continuing to list it on accounts. It took multiple calls and talking to supervisors, but I finally got them to stop.