請幫忙! - 天上掉下來的欠債

上周收到collection letter from XXX(goole checked shows a law firm in NY)
Re: CITI BANK　　Acct: ４ＸＸＸＸＸＸＸＸＸＸＸＸＸＸＸＸ
amount: $691.xx

Called CITI CARD collection dept, was told this acct was sold and no further information available.

Called collection agency XXX, was told we need pay for it otherwise will see us in court for an additional $2000, the only other information we got from them was it was opened 1996 - 2003.

My hu*****and and I don't recall anything about it. Since it is under my hu*****and name, I just requested credit reports from three credit Bureau and this card was not on his report.

What should I do know! Thank you for advice!