STIR/Shaken is on the way where in the next couple of years, you’ll have assurance of the source of calls to you. If you have an iPhone, you can set your phone up where if the number isn’t in your list of contacts or you haven’t made an outbound call to it, the call goes directly to your VM. Cuts way down on garbage calls and very few of them will leave a message.
My method is to hav one ringtone for my favorites phone list, one for my contacts list, and the default ringtone for everything else.
That way I can tell just by the ringtone if I can ignore it or not.