I actually tried being a collection agent after I graduated from college. I worked for a collection agency that took over delinquent bank loans. We were trained to distance ourselves emotionally from our clients. Everybody I contacted had a very sad story to tell, but I had to remember this wasn't a social call. That person entered into a loan contract, and the other party wanted the money. If the client wanted these collection calls and letters to end, he or she needed to make repayment arrangements.
I never resorted to threats or high pressure tactics myself, but the collection agents with the highest success rates had no problem using strong language and persistent contact. I finally had to leave the job after a year because I couldn't leave work at work.