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.
What’s more annoying than being unprepared? Doing business with someone who is. In other words, don’t wing it — practice your script, speaking slowly and annunciating each word.

The right way to leave your phone number is to start saying your phone number with your area code and then take a 2-3 second pause while the person listening to your voicemail message gets a pen and paper. Then, continue with the next 3 digits, make another 2 second pause, and then say the last 4 digits. Then, slowly repeat your phone number again.
Expanding on the simple approach above, let the caller know who they've reached. This will not only eliminate people leaving voicemails by mistake, but it will reassure the caller that they have called the number they intended.