Voicemail: How to be in when you're out

One of my friends has a very busy career and spends a lot of time travelling. She considers her workplace to be wherever she is doing her thing at any particular time, however she is not always free to answer the phone. To make up for that she is very diligent about her voicemail service. She doesn't simply note messages that have been left, she goes further than that. My friend manages to be in when she's out by updating her outgoing message frequently to share information on where she is at, what she is doing and when she is likely to return the call. She records that she checks her voicemail regularly and promises to get back to the caller quickly. And she does. She uses voicemail to maintain client relationships when she is elsewhere.
So here's a squeeze:
Many people hate "talking to a machine" and hang up without leaving a message. How can we reduce some of that annoyance and encourage callers to leave a message? One possible answer is to use some humour in the voicemail message to make things a little more personal? A way of being there, when you're not. A way of being in when you're out.
Depending on your business too much humour may come across as inappropriate and unprofessional so the trick is to sound personable and approachable.
My favourite message is one used mostly for internal calls although I sometimes use it on my mobile phone.
It runs: Hi this is the voice of N#### N##### , the rest of me isn't here at the moment so please leave your name, number and short message and I'll get back to you." ;-)
I have a way to go to reach my friend's voicemail practice and in the meantime, if I am out, I want all my callers to get a friendly greeting that encourages even the most ardent machine haters to leave that message. Common to both approaches is the professional courtesy in returning the call.
That's really how to be in when you're out.