With calls, drop any non-prearranged calls, period.
Over a few years I burned that approach into my parents. It was tough, but worth it.
My mom used to know what to do, but with dementia, she lost that ability. We now have a call block on her landline, so only a small whitelist can get through.
That's a good approach, and I do this too actually, but some people's job or occupation requires them to take calls from unknown numbers. And some people still use landlines without caller ID.