I would have thought the chief limitation in an operation like this would be that your profits are (presumably) Pareto distributed, so 80% of profits come from

I would have thought the chief limitation in an operation like this would be that your profits are (presumably) Pareto distributed, so 80% of profits come from 20% of victims. This would make the operation more like mining Bitcoin than serving lunches. One thing that supports your theory that the margins are slim, however, is that there are crime syndicates kidnapping people to work in these call centers. If the wages didn't represent a significant marginal cost, they wouldn't need to risk attracting international attention by kidnapping people in the first place.