I hate talking to people when I am in the bathroom, but at least I know they are in the bathroom. Today i saw a person peeing while talking on the cell phone's wireless headset. I hope at least he warned the person. I know personally when it's my bathroom time I need to pay attention. One tiny break of focus and watch out shoes (and I like my shoes, even though they hurt my ankles a little bit. why do you hurt me so, shoes? Why?).
And there is that moment of fulfillment when the task is completed. You can't hide that. People can sense it in your voice even if you don't make a sound. Now the person on the other end of the phone has to hear that? That is uncool.
There is nothing so important that you can't put it on hold to go to the bathroom. Just ask Tycho Brahe (depending on which report you believe. If you believe the Mercury thing, then ignore the reference. If you don't know what I'm talking about, read a book!).