Medicare Advantage is instead of Original Medicare, not on top of it. You cannot pair it with a supplement; a Medigap supplement plan cannot legally be sold to someone on Advantage. The thing that "picks up the rest" is the supplement, and it only works with Original Medicare. Advantage brings its own network, its own prior approvals, its own yearly cap. Which one is right for you is a question for SHIP, the free state counselors who are not paid by any plan: shiphelp.org.
Whichever kind of plan you have, if the bill does not match the notice, upload both. BillFighter finds the gap and writes the letter.
Check your bill — freeThe check is free. The letter is the paid part: you print and mail it yourself, or we send it by certified mail for you. No account needed to start.
This is the general rule, in plain words. BillFighter is not a law firm and does not give legal advice. If you have been served with court papers, that is the day for a lawyer or legal aid.