ER bill sent to collections because doc office took too long. what to do?

Go back to your insurance company with the note from your doctor and get them to pay the bill. Then have the hospital pull it out of collections. Worst case, I would pay the $2300 and then take the doctor to small claims for it and the damage to your credit. Medical collections can usually be removed from your report once paid due to privacy concerns so worst case, when it's paid, you may be able to dispute it and get it removed. Good luck!