They can't share any details about the debt or even that there is a debt. All they're allowed to say is "Have your friend contact me." Is that what constitutes threatening a reputation these days?
It sounds easier than it looks, but that's how you get your money as a debt collector. The ones you can't find are probably just as like to be no shows in court. Once you have a greviance, it's easy to garnish. All that involves is contacting their employer.
In most cases, any amount under $10,000 is not going to be worth going to court over. You will pay lawyers that much just to get the case to trial.
They can until someone takes them to court over it.
Having inherited the phone number of someone who owed money, and dealt with the fallout from that, I can personally testify that collectors do not consider themselves bound by the law unless/until a court fines them some obscene amount of money.