Product: Credit card

In XXXX of XXXX, I opened an account with Citibank. It was a twenty four month, interest free account. I made a single purchase in the amount of {$4100.00}. I set this up throughmy credit union and paid this amount back in XXXX $ XXXXXXXXpayments, and XXXX payment of {$160.00}. This paid theaccount in full, in less then the required time for it to keep itinterest free. In XXXX of XXXX, I sold my home andmoved into my XXXX. I lived in this at different places until I bought a house in XXXX, in XXXX XXXX. During this time, I did not receive any statements from Citibank. I was notaware of any problem with my account. When I requested an updated credit report, in XXXX of XXXX, it came backwith a delinquent account on a Citibank card. I contactedCitibank, to find out what this was about. They was not much help. All they could tell me was that I owed {$1500.00} plus late fees, and that it had been turned over to collection agency. I disputed this and asked them to send me proof of this. They wanted proof that I had paid my account, as I had stated. As my account was paid through my credit union, bill pay service, I contacted them and they sent me all therecords on that bill payment, showing that I had paid the full amount of the original bill. In the mean time I received account statements from Citibank, Showing that a charge was made to this account, for the above amount ( {$1500.00} ), in XXXX of XXXX. I have contacted them several times, tryingto get this straightened out. I have sent them the proof of my payments, but they claim that this is a new charge. I ask themhow the charge was made, by phone, in person, or by computer? I also want to know how a charge was put on that account, as I still have the card they sent me and it has never been removed from the paper that it was sent on, oractivated. It plainly states on the sticker, on the front of thecard, that it can not be used until I call the number and activate it. The card has been in my possession in myfile cabinet, and never used, since I received it. Their answer to this is that it dose not always have to be activated thatway. I sent them all of the information, and a letter disputing this charge, on XXXX XXXX, XXXX. I received a short note back from where I had sent the letter, stating that they wereforwarding my letter to the appropriate area for investigation. 
That letter was dated XXXX XXXX, XXXX. XXXX the XXXX, I received a collection notice from XXXX XXXX to collect {$1900.00}, on this bill. I called Citibank, and ask what was going on, that I had not heard back from them about my disputing this bill. They stated that they haddetermined that this was a valid bill. I have called theirsecurity ops department, and talked with a lady that wasgoing to talk with her supervisor to try to get me some help on getting this straightened away. This happened on XXXX XXXX. As that was a Friday, she said that I may not hear back till Monday. I waited until Wednesday XXXX XXXX, Icalled again. Could not get the same lady, but anotherlady was going to try to help. I have called her several times, and she does not return my calls. Can you please try to help get this cleared up?