Not lighthearted really, but I have a friend who would leap and answer his phone when he was mid-sentence in a conversation with me. Maybe it's a generational thing, I'm old but he's older, and my dad used to run off and answer the phone at home also.
The last time of many he did it I decided I needed to do something about it. His phone rang and he said "do you mind if I take this?" to which I said "yes" but he didn't listen to what I said, just assumed I was OK, and took the call. Which I had to listen to because we were chatting in a pub until he stopped talking to me.
At the end of the call I said something as politely as I could along the lines of "did you not realise I said 'yes' in answer to your question and I find it exceptionally rude that you break your conversation with me to talk to someone who just happened to call you".
He got the message, and now he's much better about it.
Don't get me wrong, sometimes someone important does call, but my book of manners tells me to apologise in advance to my companions, explain that I might get an important call and apologise for having to take it. For example, I might be waiting for someone to call me to confirm something important has happened.
But if I get an otherwise unexpected and unanticipated call when I'm talking to someone, I just let it go to voicemail, that's what it's for.
I was once at the cheese counter in Fortnum & Mason and chatting to the man about cheese when someone - actually the same miscreant friend - called me, so I answered to the effect of "sorry Bob, but I'm busy, I'll call you back" and the cheese man described me as his politest customer of the day for not ignoring him.
I don't know, it just gets me and is something I find incredibly rude. A bit like queueing up for someone behind a desk who starts talking to you, then their phone rings, and they answer it.
My dad said "it might be someone important" to which my point in response was "who am I, then?".
It was difficult working out what to say to Bob but it obviously worked because we're still friends and he doesn't answer his phone when he's chatting to me any more! I think the fact that his girlfriend confirmed that I was right in her opinion when he subsequently asked her what she thought probably helped also.