Continuous Calls on my mobile number

For the past month I have received at least 3 phone calls a day from 951-246-5500 Paetec Communications/Windstream Business ISP.  I have blocked their number from calling my number, I am also on the National Do Not Call Registry, how are these people not subject to the Do Not Call Registry restrictions?  The caller never leaves a voicemail message, I have never tried calling them back as I dont care what they have to say.

Sounds like this company uses different names/phone numbers but has the same tactics.  SCAM