Absolutely you are not being oversensitive and that's a very mind-game-ish and judgmental thing for your therapist to say. It's your session and you deserve her attention. An infrequent emergency is one thing, especially when they've informed you about a possible need for contact on their phone, but a constant activity like that is messed up.

I would not be able to tolerate a therapist having to constantly check their phone and certainly would not tolerate one attending to personal grooming or playing on their computer. My therapist did similar things (doing paperwork while I worked on a relaxation exercise, etc.) and I walked.