> The victim should either not recognize the voice as that of the relative or notice an obvious accent. <
The scammer is just rolling dice here. He hopes that if he makes enough calls, eventually his voice will at least somewhat match that of a relative.
Evidently, it works often enough.
Side point: I’ve read that some of the more sophisticated scammers troll social sites like Facebook to record voices. If they can get a big enough sample, they feed it into a computer program. That program then generates whole sentences, whatever the scammer wants to say.
Give grand kids a code word. If they don’t know the code word it’s not them!