I've had callers swearing to take me off their list without doing so. I imagine their individual performance metrics relative to those of their co-workers would

I've had callers swearing to take me off their list without doing so. I imagine their individual performance metrics relative to those of their co-workers would benefit by having you waste their co-workers' time. In that sense they're incentivized to facilitate the public service you seek to provide. While you have them on the line, could you perhaps get them to consider whether their co-workers are deliberately screwing them over by recommending you to them? Even if they're not in a position to change jobs, you could at least enlist their help in damaging the organization's morale.