A Get Out of Jail card
I suspect the reason a lot of people take calls while in meetings is that they are hoping the call will provide them with an excuse to leave.
Most meetings are irrelevant to most of the participants, serve little or no purpose (except to share responsibility for any decisions made - thereby muddying the waters about who's to blame for any cockups that arise) and have far too many attendees: most of whom know nothing about the topics on the agenda - if there even is one.
So yes, it is rude to take calls during meetings. But it's also rude to require the attendance of so many disinterested and unnecessary staff. So many meeting organisers consider the number of staff they can pull away from doing proper work as some sort of ego trip or empire building. A well-timed SMS can be the best way to get back to doing something useful.
Oddly, I have noticed that call-taking is a feature of the lower echelons. Among (truly) in control individuals: Cxx types, they hardly ever take calls while doing something else. While this may be because they have professional gatekeepers guarding access to them, because nobody has the
balls fooishness to interrupt them or simply because they are better at organising their time, I cannot say. I do get the impression that they consider being interrupted a sign of weakness and lack of control. And of course, no other attendee would consider cutting *them* off to take a call. Maybe there should be a CEO present at every meeting?