Unfortunately, Monk and his colleagues don't provide the final answer; more research is called for. But the problem seems to be that people pay more attention when they hear only half a conversation. It's apparently easier to tune out the continuous drone of a complete conversation, in which two people take turns speaking, than it is to ignore a person speaking and falling silent in turns.
Also, from the readers' comments on this article by Nielsen:
[Why Mobile Phones are Annoying (Reader Comments)]Also, now that I live in Finland (I'm from Massachusetts) I see a big difference between the US and Europe. Europeans tend to be more quiet and discreet in public (I wince at how loud US visitors speak when we're out). Mobile phone etiquette here is relatively well evolved. For example, people always set their phone to meeting mode when in a meeting, check the caller ID when it flashes, and quietly excuse themselves if they need to take the call. Finnish trains have "mobile phone booths" so you don't disturb fellow passengers. I rarely hear the "mobile shouting" that is so common at home.
We Americans are generally loud, but maybe there's hope for gradual evolution of mobile phone etiquette over time. Perhaps higher feedback volume would stimulate this?