I think this study is illuminating.  Partly because I don't take cell calls inside because it is virtually impossible not to talk too loud and partially because I can't stand listening to half a conversation anymore than the next guy...and here's why.

A new study has confirmed what we all already knew: that those annoying people on their  cellphones are very distracting. Researchers found that people were more bothered and found it harder to concentrate when they overheard a person speaking into a cellphone than if they overheard two people nearby talking.

The agitation caused by phone calls is mostly due to the brain’s need to fill in the blanks, drawing your attention because you’re trying to put the one-sided conversation into context