Telemarketers are annoying. It was a major reason I got rid of the landline at my last house (along with realising that it was cheaper just using a mobile). They are one of the reasons I only usually give out a number which leads to an expensive-to-call electronic voicemail box, which then e-mails me the voicemails as MP3s. But there is a way to deal with them.
Theoretically, the best way, as with spam and with annoying TV adverts, is not to abuse them (though that can be funny). After all, the telemarketers are just doing a job. It's not a great job, but it's slightly better than nothing. We don't blame people who work in factories which print porn for the consequent problems.
Theoretically, the best way is to stop people buying from telemarketers, or from internet spam, or products that have been advertised with annoying TV adverts. If telemarketing was not financially viable, people wouldn't do it. Ditto with spam. Ditto with annoying TV adverts. Ditto with door-to-door salespeople. Blame the idiots who buy from them.