I haven't read all the posts here, but might read them after this.
The do not call register, which I have been on for about 6 years has been great for us, even with our silent number.a number that has been silent for 15 years, we still do get the occasional call from a foreign agency.
From your OP, I don't know if it is overseas calls or locals that's getting your attention.
Essentially, what occurs in these call centers is a computer generates a number, it calls you and then one of the available operators picks up the call. The operators work on an incentive bases, which is standard practice across any industry now days.
There can be a short paused between you picking up your phone and the operator answering as the call needs to resolved at their end, it is long distance and it is VOIP.
Do you have local junk calls? If so, maybe you or someone in your home have released your number to a marketing agency. Have you subscribed to any news papers or competitions lately?