DSTV harassment

I am permanently being harassed by DSTV, my phone rings 3 times a day, everyday, and I keep blocking each new number that they are calling from, the more I block the numbers, the more calls keep coming through from different numbers, but I also can't tell them to stop calling me, when I answer the call, there is no one there and the call drops and when I call back on the same number I can't get through as it says "Line Busy". I have never been with DSTV, I have no idea where they got my number from, but it just never ends and has been going on for 3 months now.