It's a Mystery to Me

I'm sure someone else in Japan must have asked themselves this question before, but: Why, in Hollywood movies, when people answer their cell phones, does it always seem like they have no idea who's calling? Even if it's their wife or husband or best friend or whoever, they look at the damned phone and just say "Hello?" I swear, I must have seen this happen least a hundred times, and every time it just seems weird and strangely unnatural to me.