It’s hard to know if your health insurance plan is as good as advertised. You pay a monthly premium to access a network of health providers. But call the numbers in your provider directory, and you’re bound to find ones who can’t — or won’t — see you.

These errors are at the heart of a ghost network. Some providers have moved, retired or even died; others left insurance networks because of low pay and intense scrutiny . Even though these providers no longer accept your insurance, their names may remain in the directory.

When that happens, policyholders are left to believe that the plan has more options than actually exist. “Any inaccuracy constitutes a ghost network,” said Abigail Burman, a consumer protection attorney who studies provider directory errors. “This is basic information.

It needs to be right.” Insurers’ failures to correct these errors have led to dire consequences for people seeking mental health care, as demonstrated by a recent ProPublica investigation of one man’s months of struggle to access treatment . Because of the widespread nature of ghost networks, some policyholders are more likely to pay out-of-network costs and face a greater chance of treatment delays — if they get treatment at all.

ProPublica spoke with experts, clinicians and advocates to understand the challenges posed by provider directory errors. They all suggested specific ways for policyholders to navigate a ghost network. How much do insurers know about the errors.