You can ask your admin to change the greeting language for everyone in your organization. Your admin should see this topic for instructions: Change the default language for greetings and emails.
You have stated your calls-to-action. Now you have to make sure they know the process to follow. If you want your customers to leave their voicemail messages, you have to tell them from the start, “Kindly leave your name, purpose of your call, and how to contact you.”

26. Hello, you’ve reached [your name]. I’m either on another call, on a top secret mission to Mars, or I’ve just stepped away from my desk for the day. Leave a message after the tone and I’ll call you back tomorrow or in about seven months.
e. Never Assume Anything: Phrases like “You Know What To Do,” “Sing Your Song at the Beep,” and others mentioned above are awful to leave in your greeting. For the sake of universality and comprehensiveness, NEVER assume the caller knows what to do. Lay it out clearly. f. Leave a Message: This phrase, by itself, will not do. It’s imperative for users to identify themselves in their greetings. Callers need to know they’ve reached the right person. g. Disregard Lethargy: If you’re not excited about your greeting, why would anyone else be? Never display a lack of enthusiasm in your greeting as it could turn callers off to both you and your business. h. Speak Clearly and Never Slur: Callers need to understand your every word; therefore, mumbling, slurring, and all other detractions of speech should never be recorded. d. Be Creative Without Sacrificing Quality: Callers know how voicemails work–i.e. leave a number, message, etc. While you want to be clear, it’s important not to be contrive or redundant with your message. Creativity can help users to differentiate themselves, as well as intrigue callers. While users should avoid the tropes of creativity listed above, it’s definitely good to think outside the box. That being said, scripting and practice can help users to experiment more with their greeting–ultimately allowing for more unique and creative approach. e. Speak With Diction: It’s important to present one’s self as an authority without alienating callers. As such, it’s crucial to articulate and speak with clear diction. “ if your voice recording has you stumbling over words and speaking haltingly, it does not convey confidence and competence,” states Ron Sellers of Grey Matter Research & Consulting. Remember, this greeting represents you; therefore, you want to appear collected and professional, as well as welcoming. To do this, one must carry themselves well through their recorded message. f. Account for Timeliness: Your message should be concise. No caller wants to be sitting through a rant/diatribe of redundant statements. Your greeting should flow without dragging. Inversely, one doesn’t want to be terse, either. Engage callers with a simplified approach laden with creativity. h. Account for Quality: Aside from speaking clearly, users want to eliminate any noise in the surrounding environment. The quality of the greeting is just as important as what’s being said in the greeting itself. As such, one doesn’t want to undermine a great message with poor quality. i. Courtesy, Tastefulness, & Tact: This is pretty self-explanatory and straight forward–NEVER be rude. Being light-hearted and humorous is very different from being obnoxious and/or abrasive. Again, these tools can be helpful if utilized properly, but not everyone perceives humor the same way. So play it safe. The last thing your voicemail greeting should do is offend a caller. k. Provide Options: if you’re part of a bigger company, it might be good to offer caller options. For example, allow a menu to defer callers to a colleague or co-worker in your absence. This can help show callers you care about their well being. Another option might be offering different modes of communication–i.e. email, fax, etc. In offering users diversity, contact may be much easier to maintain.