The odds are high that you will often find yourself leaving messages during your job search. The good news is that most line managers review their messages personally (versus having someone else screen them, unless they are President/CEO), so you have an excellent opportunity to plant the seed for a next step connection. Here is the best message:
"Hi, _____ (target's first name), this is _____ (your first name/last name). I can be reached at _____ (your phone number) between _____ and _____ today. I look forward to talking with you then."
The only modification of this is when you are calling based on a direct referral. Your message would then be:
"Hi, _____ (target's first name), this is _____ (your first name/last name). _____ (referral name) asked me to call you. I can be reached at _____ between _____ and _____ today. I look forward to talking with you then."
Then hang up. Short and sweet. This is not the time to give your full life story from the birth canal to the present. You merely need to set the hook for a callback, nothing more, nothing less. If you make the mistake of making your pitch on voicemail, you will lose your chance to respond interactively to their specific needs. With minimal information given, the manager will often feel obligated to return the call. Who knows? You may be a customer or supplier phoning them. In 50+ percent of the cases, they will at least attempt to return the phone call.
When you leave your name and phone number on voicemail, speak slowly, as if you were expecting the person on the other end to be taking down the information. Spell your first and last name. Repeat the phone number. Not as if you are talking to a second-grader, but as a matter of courtesy to make sure the recipient is able to write down the key information from the message. This raises the perceived level of importance attached to returning your call.
If you get no response to your initial voicemail message after at least three days, call again and leave a more detailed message based on your Thirty-Second Elevator Pitch. Give a quick synopsis of who you are and what you can provide to a potential employer. Ask for a return phone call to further discuss the employer's needs. Keep trying until you do get through, although extend the number of days between each subsequent attempt.
Please, please remember that you definitely need your voicemail set up on your side to field calls when you are not available. Some college students do not set up their voicemail, expecting a text instead. Most managers will give up after no more than three failed attempts, sometimes even less. Almost all managers have grown accustomed to "phone tag" and will gladly pass the baton back to you, possibly even giving you their direct line and the best time to reach them.