I think people are being a bit too generous with the excuses about hectic schedules and all that. Yeah, things have changed, but it's more because nobody's giving it the priority they used to. Your grandparents had busy lives too, but that was sacred. Now it's super easy to fall into the phone trap because it's right there and nobody says anything. The difference is that back then there was no alternative, and people had to actually deal with each other at the table.
I get that you've got to be intentional, but it doesn't have to be some fixed day of the week - that can sound too forced and people will rebel against it. What actually works better is just keeping those bloody things away from the table during dinner. No exceptions. It'll be weird at first, like when I took my phone out of my pocket while eating and didn't know what to do with my hands, but then people start talking and it just flows naturally. After a few weeks nobody even thinks about it.
The thing is, nobody dies of hunger if they can't check their phone for an hour and a half. But you've got to enforce it without being all "you're all addicted," because then it just becomes a fight. Do it naturally, like "check out this amazing food my mum made," and it'll happen. My family's doing this now and it's way more chill.