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?
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.
And how do you define usefulness for humanity?