As a 'digital nomad' who works for a company that has employees all over the world, I try to respect the time zones of others. For instance, if I get online at 8am to start working, I don't expect anyone who lives three thousand miles away to hop in a Zoom meeting until it suits both of us. Can you imagine what a nightmare remote work would be if there was an expectation to adapt to someone else's time zone? I know a lot of people have to do this to an extent, but it's usually within reason.
Calling someone to talk about work when you know it's dinner time for them is just rude—and that's exactly what happened to u/WritesForDough42's father-in-law back in the 70s, long before you could simply ignore an email. His client, who lived in a time zone three hours behind, kept calling after hours at the least convenient times. So OP's father-in-law decided to do the same and ignore the time difference right back.