I also suspect that a lot of spam calls are coming from within the prison system - not prisoners trying to run scams off payphones, but actually working from a phone bank at a prison for a nickel an hour. It's the only thing I can think of that explains their dogmatic adherence to the script in front of them. Free scammers from overseas will start insulting you once you're onto their scam; free scammers from the U.S. will typically just hang up and try again; but if you're a prison worker, the punishment for going off script is much worse than just losing your job.
Wish I could prove or disprove my theory.