Passing the buck on listing on credit bureau

MBD listed an account I am paying through them as default on one of the credit bureau, after I requested a change on the amount to pay for one month, and they accepted this change. I have since stuck to the agreed repayments, but yet they filed a default on this.

When I contact them they are giving me the run around saying they are not the ones who filed the default, and they can't explain why the default is there while they are the ones with the debt.

Now their phones are no longer going through, citing that they have high call volumes and would call you back, which they never do.