I'm reminded of the old story about the person who thought the person in the next stall was talking to them. They had just sat down to do business when they heard the person next to them say, "Hey, how are you?" So they answered, "Fine, how are you?" Then they heard something like, "What are you doing?" and they said, "Oh, just taking a break" or something like this. It goes on a few more lines before the person says, "Look, I have to call you back. The idiot in the next stall keeps trying to answer my questions."
I personally don't have issues with the person in the stall next to me talking on the cell phone. However, I don't think public restrooms are intended as personal offices. I would put this in the same category as reading a book while long after you're done with your personal business. I might do that at home, but in public I think you need to evacuate the stall as soon as you've taken care of your basic needs.