I have never called between sessions, and I don't like talking on the phone, so I wouldn't unless it was an extreme crisis.
If I email(which I do almost every week), she will email back, but sometimes not for days later. I have tried to keep from emailing her from this reason since it just makes me anxious waiting for a reply. Sometimes I will also email her something I want to talk about in session but wont say face to face, she won't reply to those.