"We designed the default voicemail greetings to all users to standardize the feature across all Microsoft services." through. First, having a standard voicemail greeting cannot possibly work for someone who uses Skype for business. Or whose computer is set up to work in one language but expects callers using a different language.
Typically, most of these services are uniform—i.e. include the same tools and features (send a script, they record it in a tone you approve, they send it as a file, you supply any feedback, etc.); however, there are some services that go above and beyond. For example, VoiceOverPro, is a single voice professional who records 30 second voicemail messages based on users’ scripts. This is standard; however, users also have options like adding music, recording up to 60 seconds (or 150 words), rush, 2-day turnaround, and even same day delivery.

You can’t call someone back if you don’t have their number. It seems basic, but it’s easy to forget. So a quick reminder can go a long way.
Hello, you have reached the dental office of Dr. X. We are busy assisting a patient now, but will call you back as soon as possible. Please leave your name, reason for calling, and your number. If this is an urgent enquiry, please call 911. In this case, the dentist is giving a reason for not being available and they’re providing a way to prioritize the patients who need the most help. They’re also leaving an emergency number for the most severe of situations.