Credit card complaint not resolved

My credit card account was closed. The last activity was on the 12th of October 2015. All was paid up and I was waiting for them to close the account. On the 15th of December 2015 a charge back appears. I was not informed of any such actions. I could not see it on my internet banking for the account was closed. I saw it on my cell phone app. I started phoning in December 2015 to find out what it is and why. All I got from the call center was that it was escalated and they will get back to me. I contacted them in January and beginning of February and I got the same answer. Now in February they started adding late payment and interest. They still did not get back to me on what it is and why I must pay for it. The only charge back query I had was resolved +/- 2 years ago. They keep on phoning me on when am I going to pay, but they do not know what it is. All the service requests is on their system, but they still keep on phoning whilst they know their is a service request hanging.