You might also consider giving them the option to skip your voicemail message completely by pressing another key (again, check with IT). You also want to make callers aware what you will return their call, instead of telling them that you will try to return their call--and let them know when you are likely to do it, if possible. If necessary, provide additional or emergency information in your voicemail greetings. My dentist, for example, lists his cell phone number for emergencies.
As greeting messages should reflect your mood and your personality, they must be recorded with utmost care and with the right choice of words. Here are a few examples which you can make use of while recording your own greeting messages.

You can’t always get to the phone when clients, partners, investors, or colleagues call you. When this happens, a professional voicemail greeting helps you make a positive impression and tells them what to do next. There are a variety of ways to convey your message. If you’re not sure what type of message to record for your own voicemail box, here’s a definitive guide to get you started.
d. Utilizing Ambiguity Over Clarity: When setting up your voicemail greeting be direct and to the point. Tell callers your information, a brief greeting, and direction—i.e. “Hi, this is Jim Shamalam (from Iron Industries). Sorry I can’t take your call right now. Please leave your name number, and a brief message and I’ll get back to you as soon as possible. Thank you.” This is ideal, as you inform callers and let them know what they should do to ensure a return call. A lack of direction can lead to callers leaving incomplete messages (lacking contact or other information) or even callers hanging up without leaving a message altogether.