Wow. I can understand your frustration. How annoying to be in the middle of an important call and have your phone shut down at a crucial point.
The most common reason for random shutdowns is a weak or damaged battery. In the simplest scenario, it’s because you haven’t charged it recently and after it gets a little more juice, you’ll be good to go again.
When it seems like your battery isn’t holding a charge like it should, there could be a problem with the battery itself. Here’s a quick way to tell if the battery is probably getting ready to bite the dust. Remove the battery and look it over. Is it bulging or warped? Does it still fit snugly in the case? Now set it on a flat surface. Does it wobble or rock back and forth? If so, your battery has probably seen its better days.