It might be a short voicemail message that you’re recording, but you do want to make sure you hit all the bullet points. It also helps to speak eloquently over the phone and that works better when you are prepared with what you’re about to say. You are trying to sell yourself in this message so put your best foot forward. If you practice and write down what you’re going to say, you can get rid of all the awkward pauses, ums, and stumbling over words.
Business professionals are also encouraged to keep their voicemail organized. Delete any messages from the office phone that are no longer necessary after calls are returned, and write down any phone numbers that are needed at the time the message is listened to. If modern voicemail service is utilized, such as that supplied by Voicemail Office, in which voicemail and fax messages are received in a staff member’s email inbox, the individual should write a short explanation in the body of the email if forwarding any voicemail messages to other staff members.

But in the past few years, its use has been in decline. And some offices have opted to get rid of it altogether.
Context does matter here. If the caller would reasonably expect you to answer the phone (such as if they were calling a store or other place of business), an apology for not being able to get to the phone makes the most sense.