It is your responsibility as an insured to know whether you are covered or not when you obtain medical or dental services and to know for what you are covered. Further, if your divorce decree did not specifically mention dental or medical coverage, you are at the mercy now of a few entities. First, your husband may not be required to notify of termination but the insurance company have been (especially to provide some sort of coverage like COBRA at your own cost). So, you need to file a complaint with your state's insurance department to investigate the issue of lack of notice before insurance termination. Second, you can certainly attempt to appeal the issue and cost directly with the insurance company itself. As to the coverage by your employer, anyone (including your ex spouse) could have informed the insurance company, especially if it concerns an open enrollment period.