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.
Open the Phone app. Tap the Voicemail tab in the lower right-hand corner of the screen. If Visual Voicemail is not set up on your iPhone, you'll see a button that says, Set Up Now. Tap that button. Enter, then confirm a custom voicemail password. Tap Custom if you want to create your own custom greeting when someone reaches your voicemail box.

Hello you are talking to a machine; I am capable of receiving messages. My owner (your name here) does not need siding windows or a hot tub, and her carpets are clean. She gives to charity through the office and she doesn’t need her picture taken if your still with me please leave a message and she will get back with you.
As a first check, verify that you have more than two bars showing in the upper-right of your phone’s screen.