Always end with appreciation such as saying thank you. Another way you could end the call is to tell him you know he's busy but you'd appreciate a call back. Whichever way you choose to end your voicemail, make sure you smile as you say it. A mouth that is frowning has a different sound than a smiling mouth.
The business answering machine greeting you use should be fairly short and to the point, though you can use it as a way to demonstrate what your business has to offer. Your overall tone in the message should be professional and indicate to callers that though their call has been missed at that moment, they are important and you plan on contacting them as soon as possible. Within this, however, you should not indicate that you are talking to someone more important than the caller. Your business answering machine greeting should also be customized in any way you deem appropriate for your business and needs. Jotting down the greeting will prevent mistakes when recording it.

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.
Your phone system can be a powerful marketing tool for your business, and that includes your voicemail greeting. If you want to utilize this opportunity to throw in a quick humblebrag and keep callers informed of your awards and accolades, there are a few best practices, you should follow. First, keep the self-promotion brief and appropriately placed within your voicemail message. Don’t promote yourself and your products/services in the same greeting, and finally, only include current achievements in your business voicemail greeting.