How to get off telemarketers lists

What prompted this little rant was after Telefonica’s telemarketers rang me no fewer than 30 times in two days. When they rang me at 10pm on a Friday night, I snapped and called back. I am now on their “do not call” list. Want to find out how I did it? Two steps: First, ring back the company that’s calling you, on the number they called you.