should dentist verify insurance every time?

i had a dental procedure done, the dentist's office told me it would cost $423 out of pocket which I paid at time of service. after procedure done, office told me insurance had changed since they verified benefits last year, and I owe another $1000. I think they should have verified my insurance before this procedure, because if they would have told me it would cost me $1423, I would not have had it done! is it their fault? do i still have to pay?
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They verify as a courtesy. It is up to you to know the coverage your insurance. You have to pay.
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