MBD ****ming and no answer

It has been a couple of months now where I have been receiving calls from MBD. According to the sms that comes in right after the call is is regarding an account that according to me has been written off by the creditors. I have tried calling the nr on the sms as well as answering their calls but the phone either gets put down or there is just no one on the other end. It has also been listed on my credit score which is having a negative effect on my rating.