The Kaiser Family Foundation has a report on doctors who accept, or do not accept, Medicare patients.
96% of eligible physicians and practitioners are “participating providers” in Medicare. They agree to charge Medicare’s standard fees when they see beneficiaries. These doctors bill Medicare directly and the patient (or his supplement) pays the co-insurance due.
About 4% of doctors accept Medicare patients but do not bill Medicare. These doctors can also charge 15% more than the Medicare assigned fee for the service provided. But most doctors do not charge this additional 15%, which is called an “excess charge”. These doctors do not participate in the Medicare electronic billing system, so they ask the patient to pay them directly. The patient then sends their receipt to Medicare for reimbursement.
Fewer than 1% of doctors across the country do not accept Medicare assignment. They will take patients who are covered by Medicare, but they charge more than what Medicare allows and Medicare will not pay the bill.
Here is a chart from kff.org: