Debt Collector in a flash

Although the professional services are good, this groups accounting side are aggressive and uncooperative towards their customers. I had a dispute about an unexpected invoice, after remembering the Doctor saying to come back for another scan which would not be charged for. On querying Accounts told me to take it up with the Doctor. Who knows who the attending Doctor is at a Radiology practice? Is it my job to figure it out? So to get rid of the issue I paid the invoice, only to be still pestered by a Debt Collector after being "handed over". I will in future go elsewhere if I or any of my family need radiology.