I worked as a bill collector for a year or so when I graduated from college, and it was the hardest thing I ever did for a living. My trainer told me I needed to "marry" the client first. I needed to sound extremely empathetic and listen to all of their tales of woe without judgement. Once they told me their sad stories about why they couldn't pay the bill, I was told to get a little more aggressive and encourage them to come up with a payment plan they could afford, even if it was only ten or twenty dollars a month.
We had strict rules about bill collector calls. We couldn't call before or after certain hours of the day, for example. We could imply that aggressive legal action might be taken, but we couldn't use profanity or threaten physical violence, along the lines of "I know where you live and I'll send some people to convince you to pay up". The best bill collection tactic was to use a person's own sense of obligation or guilt to get them to agree to a repayment plan.