If my T answered her phone and had a conversation, even for just a minute, I would NOT like that at all! Doing that is simply not proper or therapeutic; it's downright rude, discourteous and disrespectful. The T needs to be giving you her full attention, that's what you're paying for!
I am just not sure what I would have said, if it was my first time with a T and that happened......I'm not a confrontational sort of person, so I might have simply let it slide without direct words, but I do think I would definitely have been wearing a clear frown of displeasure! If I WAS feeling direct, I might say, 'Is it your typical policy to have your cell phone with you and have personal conversations on while with a client? Because I'm uncomfortable with that.'