@99999 - that's a common response, but it doesn't tell the whole story. The people who suffer most at the hands of debt collectors are those who are vulnerable.
Example - Year ago, I got into nearly 6 figures of debt at one point over a business that went tits up. Some bad luck, sure, but it was my responsibility. However I know that the whole debt thing is a game, and you're dealing with some pretty ruthless motherfuckers. So I just played them at their own game, and came out unscathed.
(The main things I learned was never tell them anything, get hold of a mail forwarding address to use, and use a burner mobile phone so they can't harass you. Then, if you don't have any assets that they know about, they are totally powerless. If they threaten to sue you, just tell 'em to take a ticket and wait in line. It really pisses them off, but they just have to sit around and wait til you feel like settling with them. Which I did, in my own time, for a fraction of the stupid figure they'd made up.)
Whilst I was doing research on their tactics for my own case though, I came across a lot of really unfortunate stories, usually about people who had been struck by serious illness, divorce, bereavement or disability, and ended up in debt purely through bad luck that could happen to anyone.
These people were harassed at their most vulnerable. To make things worse, they were just regular folks who didn't have an entrepreneur's "fuck 'em" attitude, and were often wracked by guilt, which the debt collectors happily played on. They'd due stupid-but-honest things like tell the DCs about their savings, and what assets they owned, and in return for their honesty, they'd get screwed, and called 40 times a day until they ended up with mental health problems.