She should take it personally, because it's her personal disrespectful, distracting behavior. I would also just straight tell her, politely but clearly.

When I had my very first session with my first T, he answered the landline next to his chair and talked for like a minute in the middle of our meeting. I commented on it when he asked later if there would be anything he did that would keep me from coming back. I said frequent phone calls in my session. Then when I was just about to leave and halfway out of the door, his cell rang and answered it again, talking again for like a minute while I was standing there looking away, not knowing what to do. It was really awkward and annoying. I went back to him because there were interesting things that outweighed the phone annoyance and his phone never rang again except once when he had to let the next client in.