I missed a call, they didnt leave a message.

I have received calls from this number several times over the past six months.  Each time I choose the "opt out" selection and still the calls keep coming.  When I try to call the number to speak with an operator to remove me from their list the recording says "you are calling from a non-subscriber number, call back from a subscriber listed number"?

If these calls do not stop how can I report them to the appropriate authority?