If your internal greeting has been deleted, your external greeting is played to all callers. If you have no external greeting, the standard greeting is played.
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.)

Avoid background noise. Whether you have music playing in your office, or you’re sitting in a coffee shop, background noise can make it difficult for your customers to understand your greeting. Limit the noise around you when you leave your voicemail greeting.
An oncologist is a type of doctor who deals with patients afflicted with one or various types of cancer. His/her work involves consultation, diagnosis, application of chemotherapy and coordinating with other doctors involved in treatment etc. This is how the phone greetings for oncologist should be like: