Have a credit card with them and since I lost my job during covid last year I made payment arrangements to pay R500 on the 15th of each month and kept to that arrangement

Have a credit card with them and since I lost my job during covid last year I made payment arrangements to pay R500 on the 15th of each month and kept to that arrangement. From last week started receiving calls from MBD constantly stalking me with different numbers saying my account is handed over to them by FNB. What was the point of the arrangement then and sell their book so we end up settling even more from what these people charge. Can someone explain why I get calls from MBD after I made payment arrangement with FNB directly