[yorkietalkjilly]

That provider had a contract to only bill you the contracted fee, less any payments made by the insurance company. Ask your old carriers to send you the EOB denying the charge - front and back copy - and send the bill collector a copy with a letter demanding they abide by the terms of the contract they signed and send a copy to the network they and you used to be contracted with. Then write the network and complain and ask them to investigate breach of contract with this provider, with a copy to the provider and the insurance company. When they signed the contract, the provider had to agree to submit bills in compliance with current acceptable billing practices or to bill as directed by the contract - apparently they did neither so they are wanting you to pay anyway, despite their failure to abide by proper billing requirements. Go after them and keep copies of everything in case you have to refight this battle further down the road.