Maybe he has a kid who was home alone? There are lots of reasons that he legitimately would have to talk to someone during one session. Two sessions in a row though? I would find it most irritating. If it happens again, after he hangs up I would say something like "Wow that's the third time you have had to answer the phone during our session. Is this a bad time for us to meet?" That would call attention to his behavior and let him know that you have a problem with it.