Patient First

A few years ago I went to Patient First, an urgent care center, for care.
They usually ask you for the upfront copayment fee, but they did not, even though I insisted to pay it, which in my case it was $25, based on my insurance benefits . They told me I would receive the bill by mail, which I did but not for the copayment but for the whole service as I did not have health insurance coverage.
I called my insurance carrier and they told me they did not pay because Patient First did not submitt the claim correctly. I called Patient first and they told me they did correctly.
I was back and forth and at the end my bill was sent to collection unfairly.
I still believe it was Patient First who did not submitt the charges correctly, since it was the only time I had this issue either myself, my wife or my son who was at that time in my policy.
Until now my account is in collection which I refused to pay since It is not only wrong but unfair.