I guess the grown-up part of me does agree with you, although there's an economist in my head screaming that the more unpleasant you make telemarketing, the harder it'll be to find people willing to do it. If somebody decides to clean toilets (or burgle houses, for that matter) rather than cold-calling, that's a win for society.

FWIW, leaving telemarketers waiting shouldn't make the job more unpleasant for them, but should make it less profitable for the companies.