State your name clearly so the caller knows they have the right number. 3 of the messages I heard last month were missing his/her name.
Nobody wants to play phone tag. Skip the back and forth by explicitly telling the caller to leave their name, number, and the best day(s) and time(s) to reach them. Your clients will appreciate you not wanting to waste their time.

Voice mail allows business professionals access to messages -- even when they're away from the office. A call to any business or home used to mean one of three things -- an answer, a busy signal or endless, unanswered ringing. Increasingly, it now means an encounter with voice mail. All Career Advice Finding a job Working Interviewing Applying Resumes & Applications Seasonal Flexible Life Tips Fun Read
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.