For extension voicemail, extension users can choose how to play greetings in different presence: Default greeting: Play a greeting for any presence that doesn't have a personal greeting. Presence greetings: Play a personal greeting for each presence (available, away, do not disturb, lunch break, business trip, and off work).
Voicemails need to maintain a professional consistency that’s aligned with the entity it’s representing. That said, the structure can vary depending on the situation. There’s no template set in stone. In fact, trite and generic should be off the table. The goal should be a balance of uniqueness and practicality.

"Dear [Prospect's Name], this is [Your Name]. On [Date of Last Contact], I contacted you about [Reason for Previous Call] and as I haven’t heard from you, I can only assume you're either, one, not interested and I’m simply clogging up your voicemail with annoying spam messages; or, two, you desperately want to contact me, but you’re trapped under a fallen filing cabinet and can’t reach your phone. Please let me know which one it is. I'll gladly send help if it's the latter. Just call me back at [Your Phone Number]. Thanks for your time!"
– Sorry, I can’t be everywhere, and sitting by the phone ready to take your call, just isn’t going to happen. The best thing to do is to leave your message and not be offended that I wasn’t able to speak with you. Thanks.