Hospital being crooks!?

My husband has a $800 and something dollar balance from an injury he had in the er a few months ago. Last week, I met with the patient billing lady in her office and signed paperwork that stated that I am allowed to pay $25.00 a month on his bill and that I am required to call her with the money every month. I paid this month's $25.00 and she said she was submitting the payment agreement to corporate. My husband is continually still getting haring calls daily and telling him that they can har him until he pays the balance off. The other lady told me that she had not received a payment agreement and that she will continue to har/bother us until we pay her. She said the minimum is $70.00 and that $25 wasn't good enough. How can they do this legally when I signed an agreement and they RENEGED ON IT!