If you truly have a news breaking story, tell them, and let them choose to let you continue or not. Always be brief and to the point. Do not stalk reporters. One to two phone messages are enough. With voice mail, leave one brief message with your story, your phone number and ask them to return your call.
Reporters are people, too.
Be nice, be respectful. Develop a relationship with reporters. Work with them and they will work with you. Reporters are not your friends. They are trying to find a news story.