When someone doesn’t pay their credit card it is written off as a bad debt. Banks don’t want to hold bad debt on their books, so they write it off and it is placed as a charge off on your credit report. Then it is sold to a Collection Company the collection company adds there fee and places it as a collection account on your credit report, NO collection companies have a written contract between you and them.
Since they did not come to you and say this credit card was charged off our collection company bought it now we want to collect, they have no right to the debt. No one can collect on a debt that was written off by the original creditor on their taxes. Anyone can ask the IRS for a 4506-T showing that the creditor who had the debt charged it off.
Now that the collection agency calls day in and day out you should recend your signature from the contract with the collection agency and the credit card send it certified mail to both companies. Now you should send a debt disclosure to the collection company asking them for prove of claim, with no prove of claim they must delete off your credit report.