alt Hacker NewsArrange a secret phrase in advance- ideally generated randomly. Stick it up on the wall of the aging parent or grandparent- maybe in the bedroom, where guests are unlikely to go. Make it innocuous-looking (hidden in plain sight). Require that phrase to be said to prove identity. Reset it if it ever gets used on a call legitimately.
Personally, I require all my aging grandparents to carry a Yubikey, with an identical one always stored in a safe-deposit box. Then, on demand, they simply mate their Yubikey with a specially-prepared GrapheneOS device, open their Firefox app, and connect to the dedicated mesh network, run by and for aging grandparents. Then they run their right ring finger over the fingerprint sensor, but it must be done in a Morse-code pattern that matches their unique tattoo (I am unable to divulge the location or encoding of this tattoo). Once these conditions are met, their physical presence is confirmed by the Yubikeys of at least 2 other aging grandparents of equal or higher reputation.
It's really simple and straightforward, and there is no need to really document the workflow here, because all the aging grandparents are extensively trained and drilled every two weeks, by the aging great-grandparents who've been using this same exact system for the past 50 years.