You don’t need to say why you can’t answer or where you might be, you simply need to say your full name, that you are sorry you can’t come to the phone, and that you will get back to them as soon as you can.
The first impression callers experience. Make sure the phone prompts that welcome and guide the callers are clear, concise, and professional, with a voice that matches your brand and culture. You may not get a second chance.

Website: https://business.shaw.ca/support/how-to-use-voicemail-on-your-business-phone
In previous blogs, I’ve mentioned the importance of making a good first impression on a potential client. Most of the time, the first contact will be over the telephone. Since we are often in session, it is very likely that the first time a client hears our voice is on our voicemail greeting. This is why a professional greeting is so important. Identify yourself because you need to reassure the client that they have called the correct number. Clients will be a lot less likely to leave a message if they wonder who will get it. Use a warm friendly voice to say something like, “You have reached xxxx, licensed mental health counselor.” Tell clients what to do if they are in crisis. Although it seems obvious to us, clients may not realize that it could be several hours until we can return their call. I suggest something like, “If you require immediate assistance, please dial 911 or go to your nearest emergency room.” Ask for what information you need. Yes, the obvious name and number, but if it would be helpful to get their insurance information, ask for that, too. Give clients an idea about when you’ll return their call. Common business practice is within 24 business hours. Any extra messages or information you’d like clients to know. For example, if you are no longer accepting a particular insurance plan, the end of your voicemail greeting is a great place to convey that information.