Over riding arrangements

I have a credit card with FNB (I have had the account for over 35 years). A few months back I was in a vehicle accident and unable to generate an income to repay the full monthly installment. I contacted FNB to advise them I will not use the card any longer, however, will make monthly payments that cover more than the monthly interest and will pay more when I am in a position to do so. All payments have been made as promised. I am now being contacted by MBD attorneys (a collection company) and being threatened with legal action and extra costs. This does not make sense as payments are being made monthly as I committed to. Seems as if all FNB card is interested in is underhand deals to collection companies for them to make money and keep in business.