You don't have to pay them unless they're specific. I got a bill 3 years after being in an accident and I asked them for the documentation they legally have to provide me (itemization, the legal basis, detailed incident record, attempts on their part to contact insurance) of how they arrived at the sum. It's been a year since and they haven't given me anything.
Besides this kind of billing is banned in California now https://leginfo.legislature.ca.gov/faces/billHistoryClient.x...
The insurers just pay the in-network fee and you call it a day.