Product: Debt collection

In 2017 I was solicited for a credit card with XXXX out of the XXXX XXXX XXXX. I received the card with a credit limit of {$750.00} which I used and paid as agreed for about two years. During the second year, I had made my monthly payment after hours and my card was closed because of that. When I called I was told there was nothing more I could do and no one I could speak with so I decided to just let it go since it wasn't that much credit anyway. I continued to pay the bills each month although I had no access to the credit card. 
At some point, I attempted to pay the bill in the online system and could not get in. I called to ask again what was happening and I never received a callback. I also noticed charges on the bill that were unknown to me outside of the interest charged which caused the bill to not go down even with my monthly payments. I decided to write a check for what I believed was the balance on the account and just be done with it. Since I had no access to the account I had to guess the balance and I sent that in. That check was never cashed. I waited for a month and held the money in my bank account to no avail. 
Finally, after about two months had passed and nothing had happened to the check I just forgot about it until I got a letter from XXXX telling me the account was sold and I would be hearing from the new vendor, which I never did. 
Another couple of months passed and I received a letter from a collection agency telling me to pay the balance of XXXX XXXX dollars or it would be turned over to collection. Needless to say, I was stunned as I know I did not owe these people XXXX XXXX dollars, my limit was not even that much. I then called the agency and spoke with a very rude woman who demanded that I pay this bill, I explained what had occurred but she was not concerned at all. I asked for an itemized bill of the charges they were claiming I owed and she said that too was not her problem when I told her I was paying nothing until I saw an itemized statement. 
About a month or so later I got another demand notice from Collection for over XXXX XXXX dollars this time. I put it away waiting for the itemized bill that I had asked for prior. At some point during all of this I became ill and was dealing with that as well. I had paid this with what I thought was the balance due and even though the check was never cashed I did do what I thought was right. 
I was recently turned down for a mortgage to find that this bill is showing on my reports as severely delinquent and it at one point showed a total of over XXXX XXXX dollars owed now. I was able to get the money taken off via XXXX  but never got the SEVEN months delinquency off. 
I started writing to the XXXX of the state where the bank card was issued in Missouri, the XXXX  XXXX XXXX. I got a call from them telling me it wasn't their issue and they would send it to Az. I didn't think that would work out since the state where the bank issued the card was Missouri and today that was pretty much verified by the state of Az XXXX XXXX as they had no idea what this was about. 
I also filed a complaint with XXXX via email asking that they take this off my credit report. Since I had not heard from them in two weeks, I filed a complaint with the CFPB regarding this issue with XXXX. Just after that two weeks and after I had filed with CFPB, I received a call from a Mr XXXX at a company known as XXXX  who services the cards for XXXX but they are not the credit card company. He gave me the name of Velocity  and said this was the company they show as the account being sold to last and I should google it to find out what to do. This was the VERY FIRST TIME I had heard this name. 
So I called the CFPB and spoke to a representative to make sure it was ok to contact someone else since I had provided a complaint with them about XXXX. I was told it would be ok but if I had no success to make sure I filed a complaint against Velocity, hence this complaint.