how is a distant relative going to find anyone's cellphone number? That makes absolute no sense, since cellphone numbers aren't posted in a public directory that distant relatives might honestly peruse.
“Hey, do you Carol’s number? I want to let her know about Aunt Bertha’s funeral.”
“No, but my sister goes to yoga with her daughter. Let me put you in touch.”
To suggest they setup a whitelists where only their active set of connections may reach them demonstrates a profound lack of understanding and empathy for the elderly.
Furthermore, it's important to keep in mind elderly immigrants are usually even more stuck in the past, having grown up in nations lagging in most ways behind the USA. When I visit where my parents came from, it's like visiting a museum.
How they get the numbers is obvious, I don't think you've made any attempt to think about this. They tap into the friends network, people search through their phone books and give out numbers. It's how things used to be.
Maybe you let everything go to voicemail. Or if they're too far gone for that, you route it to your voicemail. And just pass along what's legitimate.