Frankly, I would be offended if I discovered that someone was taking a shit while they were talking to me on the phone.
Unless it's a matter of life and death, your call can wait until you're done.
Pretty much how I feel. Honestly, I doubt very much the a most of the calls you're hearing in bathrooms are a highly-skilled surgeon talking a lesser doctor through a life-saving procedure, or someone talking someone through landing a plane or off a ledge most of the time it's likely just going to be the same bullshit conversations people have on phones all of the time.
People have gotten way too damn obsessed with their phones over the years and it's gotten to absurd levels. Talking to someone on the phone while you're in the bathroom is just rude. People don't need to hear the sounds of the bathroom, you grunting to push out a loaf and you're just opening yourself up to any number of contamination possiblities.
But if you're going to do it getting mad at someone else in the bathroom doing something (like flushing the toilet) is just the ultimate in DB'ery.