I've dealt with these ACS Tony Soprano wannabes. Tried to work out payment with Citibank when I was out of work, but Citibank told me they weren't interested and sent me to ACS. They were astoundingly dumb, mostly uneducated foul-mouths, and ignored settlement offers and offers to set up payment plans. Wound up paying every other debt I had at the time in full within a few months of my troubles and telling the ACS jokers to stuff it. Guess what, disappeared anyways off my credit report within 18 months and caused minimal long-term damage.
They make the threats and harass you because if your debt is small (like mine was), they don't have the profit margin to justify suing. So, they hire a bunch of greasy junior high dropouts in New Jersey to try to intimidate you.
Whether a debt is legitimate or not, if you get contacted by these grade-A morons, strongly consider telling them to go to heck.