Medical Company accidentaly sent a bill to collections

In the beginning of February I was in a car accident that caused me to use an ambulance. I received two notices concerning payment. The first notice I returned to the ambulance company with my insurance information. The second notice I forwarded straight to my insurance company per their request since it had not been filed yet against my insurance. Since then it had been sent to collections. I called the ambulance company office and was told that they had tried to make a claim on May 5th to my insurance company, which had been denied because my insurance said it was a multiple claim (I had forwarded them the second bill). I was told by the ambulance company that this account should have been put on hold at that point, but on May 9th (4 days after the denial) it was sent to collections, without a third bill ever being sent. When I had talked to this person they said that they were going to immediately fax a letter to to the collections agency so that they would pull back what had been sent to collections due to a mistake and my credit report corrected.

Now, 2 months later, it is still on my credit report, but as paid. I had a really good credit score until this happened and it droppped my score by almost 100 points, of which it has not risen. The collections agency said it is not their problem and they can not do anything, and the ambulance aompany says the collections agency is suppose to take care of it. I am trying to buy a house soon and want to get my credit score back to normal within the next few months. What can i do to fix this and have my credit report corrected?