Try to avoid unnecessary phrases that only make your greeting longer, like “leave your name and number and what you’re calling about.” Most people know what “leave a message” means.
You don’t need to say why you can’t answer or where you might be, you simply need to say your full name, that you are sorry you can’t come to the phone, and that you will get back to them as soon as you can.

Website: https://www.att.com/support/smallbusiness/article/smb-local-long-distance/KM1200757
I have a confession to make: I haven’t recorded a new voicemail greeting since 2014. In the past four years, I (hopefully) have become more articulate, poised, and self-assured. But hear my voicemail recording, and you’d think I was still new to the work world, a little unsure of myself — and probably not an authority. Obviously I need toupdate it. And if you haven’t changed your voicemail greeting in over a year, you’re likely in the same boat. After all, a professional voicemail recording boosts your credibility, makes you seem more competent, and encourages whoever’s listening to it to continue the relationship. A relatively unprofessional one — like mine, for instance — does the opposite: It encourages prospects, recruiters, and potential connections to run in the other direction. With that in mind, I’ve written 18 unique voicemail greetings for every situation. Pick your favorite, practice a few times, then record your new voicemail.