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.)
You may also email us at [email]. If you would like one of our team members to call you back, please leave your full name, contact info and number after the beep. Good to hear from you!”

Telling a quick joke or including a little more information can keep you safely within the 20-second timeframe while letting the caller understand a little more about you.
For example, a message like, “Hi, this is Jim. You know what to do.” Well, not everyone is going to know what to do—i.e. they won’t leave you inclusive information. Additionally, they may not leave you a message at all. Aside from this, humor can again be detrimental to your message, this time lending itself to ambiguity and costing you clarity. For example, if a message read, “Hi this is Jim, sorry I can’t answer your call right now. Please leave your name and number and I’ll call you as soon as I can, but you already knew that right? Do I need to tell you what to do?” This is worse than the previous example as this is confusing and can also come across as rude and unprofessional. Complicating a greeting with phrasing like this is sure to cause some harm. e. Ignoring Personality & Identity: Don’t use computer generated greetings. Some users leave default messages (‘you’ve reached the voicemail box of 777-777-777, please leave a message). Believe it or not, even this can cause problems. Callers may be unsure if the voicemail box belongs to you; therefore, they don’t leave a message. Also, some may even be uncomfortable leaving information through a message in a nameless voice message box. As such, impersonalization can cause ambiguity, which again can hurt the effectiveness of your voice message system. This doesn’t mean you have to make an elaborate greeting if you don’t want to, just insert your voice and name so at least callers know they’re calling the right person.