There are basically two types of individuals who end up working at collection agencies -- people who are recent to the country and have no other options and people who get enjoyment out of trying to bully others usually because they are unhappy in their own life and powerless. Anyone else who tries it doesn't last very long.
As much as I think what collection agencies do is immoral and wrong the employment is commission based -- both for the company and for the employee who is calling. Being an as***le is more effective than treating people with respect. To make enough money to make the job almost worth doing you need to be rude and abusive.
A good strategy is to not let them upset you -- especially if you know that they have no power. I'd be very nice but just not interested in talking about the debt. If they are upsetting you then they know you want this to stop so if they just annoy you enough you might pay to make it stop. If you don't get upset, annoyed, and show complete indifference with a hint of mocking they realize they are wasting their time and will stop calling.

