I quickly got up to 15+ calls a day on my number by calling all scam numbers and baiting them. The ones that did not pick up, I left my number. Believe it or not the ones I baited....different people would call me back from the same "company" trying to scam me as I guess they don't keep good records. All they see is that I had called them before and tried to pay. But other people have suggested that one collection method they use is buying numbers from online surveys.
Just make sure to use the same number each time that can actually receive calls back.