In my (US-based) experience, doctor offices & the like have a place where you can fill out if they can leave messages or not (and some even have asked detailed messages vs callback messages), so that's my perspective! And that I can set different things for different numbers. So my cell phone they can leave detailed messages on any matter, my mother's house phone is detailed from billing only and otherwise callback, her cell phone is callback only. (And they also aren't allowed to tell her anything other than billing, so even if she does call them back about anything else! That was just a measure set up when I lived with her one summer & had no cell reception at her house.)
Thank you for calling ABC Company. We are currently closed for the holiday. Our normal business hours are 8 am to 8 pm, eastern time, Monday through Friday. Your call is very important to us. Please leave your name, phone number, and a brief message regarding your call. We will return your call on our next business day. Thank you.

In the former, the voicemail message shall be delivered to callers from within your company i.e. for communication exclusively for office employees. In the second case, the voicemail will be received by people calling from outside the company. Hi. You’ve reached Martha Johnson in Human Resource department. Unfortunately, I cannot attend the call now. You can leave your message after the beep. Hello. You’ve reached Martha Johnson in Human Resource department. You can either record your message after the beep or contact my secretary Henry Smith at extension 5556. I shall get back to you soon.
Of course, your phone rang because someone wanted to get in touch. They got your voicemail because you were busy or just not there. If you section off a block of your day to check voicemails, let the caller know so they can expect a time for you to return their call. People will leave more details if they know someone will check it later.