Should I call to a guy back who claimed on a voicemail that he bought my last vehicle for his kid?

I wouldn't, but that's just me. I'd be concerned that anything I say could be used as evidence the car was sold faulty or something weird like that. Whenever I sell an old car, I make sure to put it in writing that it is being sold as-is, no warranty or refund to be expected or offered. Once it's out of my hands, I want nothing more to do with it.