They're basically creating fake numbers (or spoofing other peoples' numbers), and then randomly dialing a bunch of people until they catch someone falling for it. Since it's all automated they can make thousands of calls an hour to thousands of numbers, change the number after an hour, and repeat.
There aren't too many ways to track it to the source, because the 'source' is a spoofed number so you're like blocking a pizza hut or something.