After leaving four or five messages without receiving a response, you should stop calling – at least for now. Don't expect the courtesy of a call back from a hiring authority. Unless they have a need for someone like you, it is not likely that they will call you back. Don't take it personally. If you are looking for a job or soon may be, join Tony Beshara for a FREE webinar: The Top 10 Reasons You Might Have Trouble Finding a Job (and How To Combat Them).
A positive attitude extends to your voice and how you’re speaking. At the same time, if the prospect can’t hear or understand you, all of your work is wasted. Follow the four Cs: Be Clear: Drink water, clear your throat, invest in resources like a good headset and phone. Be as mindful as possible about speaking clearly. Be Concise: Keep it short and simple. Use a good Cadence: Don’t be afraid of strategic pauses, and vary the speed of your delivery. Be Compelling: Speak with authority.

A voice mailbox is typically associated with a telephone number. When the number is called, and the line is busy or not answered, the caller hears an outgoing Greeting recorded by you and is given instructions for leaving a message.
Website: https://energyresourcing.com/blog/how-to-leave-professional-voicemail-messages/