I answered after few days of ringing today. It was a male voice calling about whether I have smart meters then I said yes I have. He then to offer better tariffs from company called "utility".
I ended the call, Saying I just moved to a new supplier recently then he was pushing to pursue for another move and said not interested.
This along with other Sheffield numbers call at least twice a day. Leave recorded message regarding "energy plan" best as I can decipher, Not the best quality. Anyway blocked just like the rest.
What gets up my crow is that you need a company name to report them, Of course they leave that part out of the message, But do ask you to press a number to reply, I don't think so.