Just in case you fall victim to mumbling or speaking softly in your voicemail message be sure to repeat your contact information at the beginning AND end of your message so that the listener is more likely to be able to pick up on the information. This helps if you are speaking quietly or if the listener was not ready to record your information when you said it the first time. Repeating your information will save the listener time as they will not have to listen to your message again to get those details.
What to Say in a Voicemail Greeting. Here’s what you should say in a professional voicemail greeting: A greeting; Your name; Your company; A simple explanation for missing the call (e.g. you’re away from the phone or are on holiday) A …

Friends and colleagues speak to each using first names only. So do people of authority. They do not call each other and leave voicemail messages asking for Mister, Miss, or Mrs. Therefore, when you call a person you want to do business with and you leave a voicemail message, refer to them by their first name only. Don't say mister, miss, or misses. Don't say their last name. Begin your voicemail message by saying only "hi/hello" followed by the person's first name. Or, you can even forget the "hi/hello" and just say the person's first name. That is how you show confidence and authority and separate yourself from weak salespeople.
Users often don’t invest enough time into their messages, resulting in incomplete, unprofessional, or otherwise under-whelming greetings. Sure, crafting a greeting doesn’t sound all that complicated; however, there are a number of pitfalls users can fall into—i.e. informality, terseness, sincerity, lack of direction, and more. While none of these sound too catastrophic, they are often interrelated. As such, they tend to worsen any problem. For example, humor can cause informality, worsen ambiguity, and weaken sincerity. That being said, users should strive to avoid ALL these pitfalls.