Do you expect her to be punctual within seconds or are you concerned about your feeling when she is seconds late? These seem like different categories to me.

I think it is unrealistic and rather harsh to expect her to be punctual to within a minute. I understand that it might make you anxious, but do you need to do anything with that anxiety other than feel it? What about noticing that you feel anxious, paying attention to what happens in your body, noticing what happens to you when she does call, and so on. Maybe then you can bring those experiences to the session rather than impossible expectations about time keeping. After all, this is a safe way to experience anxiety. You feel anxious for a minute, she calls, your anxiety is ill-founded, and you are then well placed to discuss your experience of the anxiety. Seems like a safe and controlled environment to learn to tolerate uncomfortable feelings.