If it was an important call or text etc, I'm fine with that, absolutely fine, things happen in life, but If my T did it all the time I think I'd get pretty annoyed with it in the end!
I suppose I just see it as a respect thing, like if I'm talking to someone and they keep picking up the phone I'd just think obviously what I'm saying isn't that important, and I wouldn't do it while someone is talking to me either generally!
Normally I put the phone on silent or flight mode when I'm in session! I think once or twice I've needed to keep an eye on it and said beforehand!!