Fort Sill National Bank ,FSNB, Signor reported as Joint never agreed to be a joint account Lawton Oklahoma

I was added as a signor on my girlfriends bank account. She had already had the account for some time before I had became a signor. We went into a store branch in Shreveport LA and the took my ID my Social and I signed the Signature Card and some other papers along with it. It was never explained to me that I was agreeing this to be a joint account. I had no intention of this being a joint account because I did not have any money going into this account. It was to help my girlfriend if she needed me to write a check for something if she was not available to do so. Doing some check on my credit I now see that the account was overdrawn and it was reported to my credit. I have disputed and it still remains. I called the bank they then tell me that I signed for it to be a joint account and that is not what I asked for or what I was told in the branch. I signed what I thought was the Signature card. They then tell me that they spoke with me and I agreed to the debt and I KNOW FOR A FACT I DID NOT. I do not agree to any debt over the phone and never have and never will. They then refused to play the "recorded" call to me. I was lied to by their WALMART branch employee and now I have a charge off on MY credit for an account that was not mine. I just want this removed off of my credit nothing more. This was not a joint account I was just a signor.