"I think it's unreasonable to object to a phone call unless you're in the quiet carriage. What's the difference between a phone call and a conversation?"
Two things here.
Firstly, both are annoying if they are unreasonably loud, but people on the phone tend to be louder because they think (often correctly) that they need to speak more loudly to be heard, compared to having a conversation with the person next to them.
Secondly, I think this "unreasonable to object to noise" point of view is arse upwards. You're on public transport. You're not in your own home, or in a public park or even walking down the street. You're in a space where you are surrounded by members of the public, who are often trapped in your company whether they like it or not. Even if they are in a position to move to a carriage, this means they have to physically get up and take all their stuff with them, and hope that (a) they find another seat, and (b) someone else doesn't start doing the same thing in the place they have just moved to.
By talking loudly on the train - whether on the phone or in person - or making any other kind of noise which is more than minimal and/or lasts more than a couple of minutes, you're forcing the people around you to listen to you, whether they like it or not.
People should give some thought to the people around them, as well as the general noise levels of the environment they are in, and be more considerate.
The fact that so many people are inconsiderate twats these days that it has become commonplace is neither here nor there.