How do you respectfully call someone out for bad manners in public?

I frequently find myself in situations where someone’s behavior goes against well-known norms or even common decency. I work with people who dive into their smartphones while in the middle of a conversation without excusing themselves. They will literally check out of the conversation they’re having with me in person, start texting someone, then resume the conversation with me as though nothing happened. I think this is quite rude.

Or, there are people who smoke at my community park where it’s clearly prohibited. And they play their music loudly. I know these behaviors aren’t exclusive to young people, but it seems it’s almost always young adults who are guilty. Has nobody taught them?

Every time something like this happens, I want to say something but am not sure how to do so. I worry that speaking up will offend the person or lead to an argument or make matters worse, and yet I want to say something. I think our communities are better when people respect basic norms of, well, respect. Any suggestions?

Signed,
Old Fashioned