PB is right. In fact, when someone begins to rant/complain be the First to offer that they be transferred to a manager, if possible. Otherwise, you handled this really well. All you can really do is know this has to be someone who is either chronically 'impossible' or in a state of mind that lets them think it is ok to take it out on someone they don't know. (or whoever is available) Also, if you do have a boss you are ok with, I would let them know of any particularly distressing calls (or at least make a note for yourself so you can't be blindsided be a "complaint" out of nowhere)
You did really, really well.