A business voice mail should have several ingredients that inform. Include the name of the person whose extension was dialed, the company’s name and the department. Invite the caller to leave a message; often the caller does not realize she has reached voice mail.
Cell phone voicemail greetings are generally more personal and casual than office phones. Consider a greeting like “Hello, you’ve reached [your name]’s cell phone. I can’t take your call at the moment, but if you leave a brief message, I’ll get back to you as quickly as possible.”

Hello, you are talking to a machine. I am capable of receiving messages. My owners do not need siding, windows, or a hot tub, and their carpets are clean. They give to charity at the office and don't need their picture taken. If you're still with me, leave your name and home phone number and they will get back to you.
Using your cell phone to record a voicemail message isn't always easy. Between remembering what to say and getting the right timing, it's tricky. But your phone does have a playback option, and it's easy to listen to and review your message. For some people, though, cutting off the end of a message is a funny way to trick people and catch them off guard. In most situations, callers won't know it's intentional and will think you're far less than tech savvy. Definitely not an impression I'd want to make.