Ignore them. They are an aggressive lot - as you have already discovered.
It may be that they are now acting as debt collectors for one of your debts (creditors often change the debt collectors without telling the debtor) but if so they need to write to you giving the details of the debt concerned. If they ring again and you answer the phone tell them you'll have nothing to do with them until they do that. It's quite wrong for them to suggest they can simply give you such fundamental information on the phone. What they really want is to try & pressurise you into paying more than you can afford. Don't let them.