[請教]：關於醫療賬單與追債公司

背景：

Medical Associates didn't file the medical claim on a $300 bill, although they had all my insurance information. Then, they referred the bill to a collection agency. The collection agency is collecting double the original bill amount (~$600), but refusing to explain/provide bill for the extra amount. The collection has been on/off for 5+ years. They thretened to take it to the next level (law suit, I guess)

I refused to pay, because of the total negligence on the Medical Associates part, and the misleading collection activity (more than original bill amount) on the agency part.

What can I do to stop the collection?
What is the chance of winning a counter-suit?

Your advice will be appreicaited!

Thanks!