It's Called Ettiquette. USE IT!

When I go into the little girls room to do my thing, I do not want to walk in and see someone talking on their cell phone.

Person on cell phone: "What sound?"
"Oh! THAT sound. Someone walked in and is going to the bathroom."

If I wanted random unknown people listening to me pee, I would ask to have a toilet installed in my cubicle.

The workplace bathroom is no place to be talking on your cell phone.