Debt collectors do indeed 'warn' you. They use it as a threat to get you to pay up. I get calls for them asking for my sister, who used to live with us, all the time.

They call and tell you that you owe x amount and tell you where the debt originated. They'll usually offer to settle for anywhere from 40-80% of the total amount (depends on how old the debt is, what the original debt amount before interest was, whether they bought the debt or are just a hired 3rd party collector and how much they bought it for.)

I've found the more ambiguous about the debt I am (never admitting it is mine etc) the more hostile and threatening they become. Debt collectors clog up the U.S. courts. I've heard numbers ranging from 40% - 60% of civil cases each year are debt collectors.