Product: Credit card

I held a credit card with Macy 's department store, issued by Citibank. The account was used sporadically, and always paid in full at the end of every billing period. In 2016, I am in the process of relocating. A change of address was filed with the US Postal Service, and my mail was being forwarded to a new, temporary address regularly. I received a letter from Citibank on XX/XX/2016 describing the fact that they had been trying to get in touch in reference to a Macy 's charge account, ending in XXXX. This letter was forwarded from a previous address. Since there had been no regular bill sent for this account for several months, the communication was dismissed as a marketing ploy, since no reference was provided to an outstanding balance. 
I then received a letter from XXXX XXXX , XXXX, also forwarded from a previous address, stating that they were beginning collections of an outstanding debt, and referenced a Macy 's American Express account, listing an outstanding balance of {$410.00}. Their account number is XXXX. 
Upon receipt of the referenced collection letter, I called Citibank to try to resolve the issue. Their response was that they could do nothing to resolve this issue since it had been referred to an external collections vendor. All attempts to acquire information from Citibank were met with stonewalling. The credit card in question is in an unknown location in my storage facility, along with legacy statements and records. Citibank would not provide the account number associated with this case, even though they verified my SSN and security response. The customer service representative was trained to tell us that she ( XXXX ) did not have a supervisor so we could not escalate the matter with them. She finally agreed to put a note in her system to have a manager return my call ( ref # XXXX ), which was never attempted. I tried this approach with another representative, with identical results, indicating that this is Citibank policy rather than a single bad actor. 
I then contacted XXXX XXXX , XXXX to indicate that I was disputing this debt, and received verbal verification that I had responded in this manner in a timely fashion, in compliance with the procedure shown on their letter dated XX/XX/2016. Their recommendation was to submit a message via email to XXXX, which I do in parallel to this action. The text is copied below, with identifying information redacted : Hello, We were recently advised of a collection effort by your company and require additional information to proceed. I spoke with XXXX XXXX, XXXX XXXX XXXX about my questions and concerns today and she advised me to submit this email. 

Since we are unaware of the charges, this is an official request for you to validate the debt as described in your letter : XXXX Acct XXXX Balance {$410.00} Date of letter : XX/XX/2016 To date we have not received an invoice with this, or any other amounts. Therefore, we are requesting documentation outlining all information referring to {$410.00}. We will not be able to address or resolve this matter without it. 

Please email details back to XXXX or by mail : XXXX XXXX We are very eager to resolve this situation quickly. 

Thank you very much in advance for your help. 
Name Phone Number