MBD - Adding extra cost after settling the account

I had an outstanding balance on an account which I paid off on the 16th of July 2018. I am now harassed by MBD call centre staff that there is an additional outstanding amount due to the fact that they contacting me. This is after I paid the exact amount as per their written instruction. To add to this they have not contacted me since my payment, so I am now forced to make another payment because of the calls I received 2 weeks after I settled account in full.