If she's not conversing with a co-conspirator, investigators have theorized, she could just be using the phone as a prop. "Is she talking to someone," Troxell said, "or using it to make herself look less noticeable or nonchalant as she approaches the teller?" Some might argue that people who stay on a cell phone throughout a bank transaction tend to draw attention to themselves.
My theory: she's showing off, trying to look cool. For most people, the stressful act of committing a crime in public would require their full attention. This woman is all like, I can do this and check in with my former roommate at the same time.