Unfortunately this is your responsibility.. The insurance company fulfilled their obligations by policy..it was sort of up to you to know your plan, know that it was an HMO. HOWEVER..the provider even though they really didn't do anything technically wrong, also missed this..what I would suggest is seeking their help with an appeal and they might even be able to do an out of network negotiation with the carrier.. MAYBE. Ultimately however the bill is technically your responsibility. There were no unfair billing practices. The liability ethically is split between yourself and the provider (ETHICALLY speaking, not legally speaking) If the appeal with the insurance company doesn't work out you might be able to negotiate a lower fee with the provider, by appealing to their sense of moral obligation for also failing to check before hand.