This is very incompetent rude and plain unprofessional. No wonder she goes quiet and runs out of things to say because every time she picks up her phone she is being distracted. She can't clearly give you her undivided attention is she is distracted.

You are paying her to listen to you and help you and from what you have said she is doing neither. I would be very annoyed with her and feel as though she didn't care enough to not answer her phone.

I would say to her in your next session that you feel it is inappropriate for her to be texting while in session and see what her response is. My t answered the phone twice in my session and it annoyed but I didn't say anything and it still bothers me now so these things will eat away at you if you don't get them out.