I would keep my voice low and steady and never raising it to their level. I'd reassure them that I am here to listen in hopes of resolving their issue and that by calming down, I will better be able to assist them. I would"own" this call and not transfer them to someone else, further agitating the situation. If it is something beyond my authority I would assure the client that I would continue to stay on the line with him if he prefers me to do so.