What contract if the insurance no longer accepts them or they no longer accept the insurance? The insurance doesn't cover said doctor, they have nothing that prevents the doctor from telling the patient as far as I see.It may have been part of their contract with the Dentist. So what can the insurance company do if a Dentist discloses that information in violation of the contract? Sue them for breach of contract. As the Judge said in Jury instructions on the law in a trial I was on the Jury for.........."something can be ethically wrong, but required by law"......in this case, a contract with the Dentist.
And the "The insurance company forbid us from talking to their patients." part, insurance companies don't have patients, they have clients and in no way has any bearing on what medical service who doesn't accept said insurance tells their patients.