i'm NOT interested!!

Clearly the Donot Disturb Registry isn't working.
I got a call from an unknown number and something told me that it would be one of those overzealous salesmen selling me some insurance/policy or something like that, which I most certainly don't want. So i didn't pick up my phone. I again got a call from the same number, and thinking it was something important i picked it up. And what do i hear??
"Hello mam this X calling from abc are you interested in..."
Me: NO, I'm not. Please dont call again.
And i end the call.
The guy calls AGAIN!!! This time:
"Sorry maam it seems there was some problem the call was disconnected!! As i was saying, are you interested in...!!"
Right..