HDFC Bank — Calls even after registering for DND

I get continuous calls from HDFC bank asking to take loan, credit cards etc.
even after registering for DND .It is frustrating as we will have other work . We cannot ignore calls from unknown numbers as we get calls from schools / colleges etc .
This is going on for years & i am totally fed up . Even after cutting the call they keep on calling again & again . The team might have given targets from bank which should be taken out .Some action has to be taken regarding this & some system should be developed to block the numbers which have registered for DND.