Of course he gets angry. He works on commission in a boiler room setting and his boss is standing over him with a whip pressuring him to collect money or lose his job. People who work as collection agents are often losers who can't do anything else. Once you understand that you can have some fun with it.
Try this. Set up a recorder on your phone. Next time you get a call from the collection agency or a number you don't recognize pick up the phone and say:
"This conversation is being recorded. If you stay on the line you have given your consent to be recorded. Now what do you have to say to me?"
The more angry you make the guy, the more satisfaction you get. You can hang up at any time.

