I have a couple questions/ starting points,
1) who made the mark on your CR was it a CA or the OC?
2) can you get in writing the dates that the office was in network and the dates they were out of network?
3) send the doctors office a written request that your chart and all documents signed by you, to be sent to you.
4) write down in detail as much information as you can remember. Receptionsist names, dates etc. Call your insurance company again and find out why they suspended that office for that time period.
Research you states laws and regulations on debt collections. Focus on the articles on medical bills for this account.
5) Once you have all of that information I would call the doctors office and ask to speak to the billing specailist. Have all of your information with you so that you can quote documents from the office and the Insurance company.
EXplain to the office everything that happened and how you were willfully mislead into accepting services from that office when they knew their office was suspended by your insurance company. Provide them with the details you find out about why they were suspended.
I would also inform them if they do not remove any marks from your CR that you will file complaints with the FTC, BBB, AG and DOB.
Chances are you will be able to get them to remove the mark from your CR if you offer them a PFD after you hit them with all of this information.
Personally, I prefer to do all my dealing with CAs via mail. That way I have proof of all conversations. If you need help drafting a letter for this I can help you. Let me know what you want to do and I will throw one together that you can add your own details to.
Now, when exactly did this hit your CR? How close is it to the SOL in your state?