In spite of signing up on the Do Not Call Registry (numerous times now) and downloading NOMOROBO (a little better luck with this)...I still get lots of unwanted calls.
So I've simply resorted to an antiquated technology (albeit seemingly vastly underutilized, if my observations in public are any indication) called...
voicemail. 
If the call doesn't show as one of my contacts, then I will check later...to see if they've left a message.
If it wasn't important enough for them to leave me a message, it certainly wasn't important enough for me...to answer it in real time.
As for the OP's comment that the number comes up as a legitimate Medicare number, I'm not sure what you can do about that...except the voicemail route.
