Hello, you have reached Dr. X. Right now, I am with a patient, so I’m unable to respond to your call immediately. If you need help with a prescription, call number Y. If you need to book/modify an appointment, call number z. Please, leave your name, and phone number and reasons for calling.
There is one very important last step. Return all of your calls as soon as possible. Prompt, professional callbacks, even if you then must leave a message, get you off on the right step toward a job interview.

Friends and colleagues speak to each using first names only. So do people of authority. They do not call each other and leave voicemail messages asking for Mister, Miss, or Mrs. Therefore, when you call a person you want to do business with and you leave a voicemail message, refer to them by their first name only. Don't say mister, miss, or misses. Don't say their last name. Begin your voicemail message by saying only "hi/hello" followed by the person's first name. Or, you can even forget the "hi/hello" and just say the person's first name. That is how you show confidence and authority and separate yourself from weak salespeople.
“Thanks for calling the desk of (insert your name) at (insert your company name). I’m either on a call or away from my desk but if you leave your name, phone number, and the reason for calling, I’ll return your call as soon as possible. Thank You.”