Of course not during the actual interview! Just that some of these visitation weekends are several days long and crammed with activities, and for the ones I've been invited to I am staying with faculty members and graduate students. I think it would be awkward trying to find a time and space to sneak off and call another school back.
This is probably a long shot, but I was curious if this had happened to anyone before. Have you ever been interviewing or visiting one school and gotten an acceptance or interview via phone from another school? If so, how did you handle it? Also, lucky you if this has happened.