Telemarketing may not be useful, but now the goalpost seems to have moved from "prove most jobs are actually useful" to "prove no jobs of questionable utility exist"

Agreed, and the same goes for the people that do "customer support" on call centers set up in basements, run by "managers" fit for a reenactment of the Stanford experiment, dealing with disrespectful customers that are angry about the crap sold to them by the same company that outsources "support" to yet another outsourcer. All while these people do nerve wrecking "work" for minimum wage, because they have to eat, until they burn out. And this is in Europe.

Useful is such a relative word.

Telemarketing is useful in some way to the person who employs the telemarketer. Otherwise he/she wouldn't pay someone to do it.

It's also useful to the person earning the salary, who then spends their salary consuming in the economy. Not everyone is lucky/brave enough to pursue what they are truly passionate about.

Finally, even if the job doesn't generate any sales, it's still redistributing wealth away from one set of people (shareholders of the company) to others in the economy. Is this useful?

Useful for humanity. Because "useful for employee" or "useful for employer" are bad measures that allow really stupid things to exist.

This job is based on lying and/or deceiving to people to trick them into buying somethng. Whoever is paying for it is actively hurting other humans. In a way it's worse than bullshit - it's a malicious job.

Are you implying that there is no place for malicious jobs in society?

I am. Before someone brings up the military - you can say that armed forces are built on malice, but the harm is at least directed towards other societies. Telemarketing is basically hurting members of your own.

Before we start, what do you define as a telemarketer?

And how do you define usefulness for humanity?