AARP Hearing Center
The ONLY time a beneficiary would encounter them (and it is rare) is if the provider did not accept Medicare Assignment and they are deemed “non-participating” by Medicare.
This Medicare link describes the “non-participating” providers as well as what it means to be a provider that accepts assignment and the ones that work only on a contract basis and thus does NOT accept Medicare at all.
Medicare.gov - Does your provider accept Medicare as full payment?
“NON-PARTICIPATING” Providers
From the link: [copy/paste]
Some doctors, health care providers, and suppliers who don’t accept assignment still choose to accept the Medicare-approved amount for services on a case-by-case basis. These providers are called "non-participating."
If your doctor, provider, or supplier doesn't accept assignment:
end copy/paste - there is more at the link ~
It may also be worth mentioning that some states have outlawed the practice of excess charges for Medicare in their state. Last I checked (in mid 2025) these states were: Connecticut, Massachusetts, Minnesota, New York, Ohio, Pennsylvania, Rhode Island, and Vermont.