Nope, she shouldn't have her phone in session at all - I honestly can't think of anything so important that would merit a therapist using their phone during session. If there is something they need to attend to, they need to go and do it. For me it's utterly unacceptable for her phone to be in and for her to reply.

Can you tell her you don't want her phone on during sessions? By that I mean no ifs, buts or maybe'S just that she turns it off and gives you her 100% undivided attention? It's just so rude, I wouldn't sit with a friend who kept checking their phone over coffee me ch less my therapist.