Most of those phone marketing services use fully automated servers to handle huge numbers of phone calls without "wasting" human resource. If the machine gets sufficient cues that they got someone that is likely to be the person targeted and likely to answer the call, they will pass it to some human usually working in very low-paying countries.
All this annoying spam may seem not only annoying, but also irrelevant in our Internet world. After all, it's much easier to just mass-mail, use various means of online advertisement, etc.
But they still use phone calls, because through a well known and pretty basic psychological effect, it's a whole lot easier to trap someone while directly talking to them than if they receive advertisement that they can read whenever they see fit or not read at all without having to justify themselves to anyone...
This "conversational" effect is so powerful, I'm thinking that as more and more jobs can be and will be automated, the jobs most likely to remain for humans are sales jobs.