We also like to socialize with our guests in the shares areas – however not to listen to their private phone calls.
But, I think, you can’t educate guests. Either they are well-educated and don’t do this – or they aren’t. Same in the tram: there are polite and impolite people.
How about listening to your favorite music in a volume that is comfortable to you (but not to public callers)?
We drastically shortened our house rules. We never had such a rule included, and would find it very strange to find such a rule when we travel.
I would focus on finding the right guests for the future. A good matching makes hosting easier.