That's an easy answer. Is the conversation more important or the unknown caller. for sure if I'm providing my time and someone answers their cell phone I walk away..... If mine goes off I read the message later. two people have a special ring where I might look in case of emergency but not answer.....i never break concentration on the conversation at hand.

If nothing else is common civilized politeness. something worth far more than any cell phone call which these days is an unblockable or blockable damn advertisement of some sort most of the time.

It is disconcerting to people to realize they are more important than a cell phone call. especially if i say...let me turn this off while we're talking.