The scam is usually a phone call from a loved one in jail or the hospital. It immediately creates a sense of panic and fully explains the random phone number. The voice doesn't have to sound like you either.
"You're my daughter? Then tell me the name of your twelth pet centipede."
It should be a simple, pink-elephant-type nonsense question and answer that you rehearse a few times in person and make sure they never share with others. “Q: What’s your favorite leprechaun? A: Rutabaga” or something. Even cognitively impaired folks can, with a few rehearsals over a day or so, retain that easily, I’ve found.
Reminding them to practice it when people reach out pretending to be me takes a bit more rehearsal, but I’ve found that it sticks reliably even then.