Dunno. It's a per peeve of mine when people take a non-speakerphone call in a common space. Trains, cars, meals, or living rooms.

It monopolizes the noise in the area and puts everyone else in some sort of awkward space where they have to eavesdrop and pretend they aren't. Sometimes I think about obviously watching a train commuter have a long and loud phone conversation, but I don't because it would be rude and aggressive.

I guess hired cars are a little different in most respects but people can still feel bad or awkward for listening in on something they aren't part of. Especially if the conversation is loud, personal, or confidential.

It's much better if the phone call is urgent, short, or important. But even then it's polite to be conciliatory as the phone call is made or picked up.