1. something you have
2. something you know
3. something you are
These three things are required for any authz.
At that point, how can you validate it, as there’s no shared secret? The only thing we had was validating childhood stories. After a preponderance of them, we accepted she was real (she refused to talk on the phone — apparently her voice was damaged).
We eventually met her in real life.
The point is, you can always use these three principles: asking relatives to validate the phone number — something you have — and then the stories — something shared — and finally meeting in real life — something you are.
I only say this sort of jokingly. Three out of four of my parents/in laws are questionably literate on the internet. It wouldn't take much of a "me bot" for them to start telling it the stories of our childhood and then that information is out there.