It would have seriously bothered me. I don't think you were wrong to be upset. I'd have reacted with quite a bit of anxiety and frustration.

I've only had T tell me one time that our session might be interrupted, and that was because of stuff going on with her family. Even then, she told me that it would only be a possible text message that she would need to respond to. She did get the text message, glanced at it, replied quickly, and we went on with my session. I was fine with that. She made it as non-intrusive as possible, and she warned me ahead of time.