I found this helpful article at the blog for the Publicity Hound.
I ALWAYS practice tip number 2. Reporter's seem to appreciate it and will tell me to email instead or call back in an hour or so.
2. Be considerate of the reporter’s time. As soon as Karen answers the phone, ask, “Hi Karen, this is Joan Stewart and I have a story idea for you. Is this a good time to talk?” If not, she will say so. Ask her when you should return the call.