I recorded my voice similar to "Lenny" youtube videos with pauses, that I found hilarious years ago. I had some success with keeping telemarketers busy for more than 20 minutes, I had to loop my 5 minute or so recording! It was fun to play back the recordings of the telemarketers although they are long lost now. They just kept calling though.
Now with spam filter and the factor I don't answer if I don't already know the caller I haven't talked to one in years, probably 1 call a day I get.
There are AI solutions coming if not already available, the app answer your phone and asked, who are you? what is this call about? It shows you the response in text and you decide to pick up or not. Might be a bit better until telemarketers start falsifying their motives, "Actually it is not your bank calling but I'd like to interest you in...". I find it odd soneone has ever actually bought something this way.