I had a colleague who simply never answered her phone or set up her voicemail. She still listed the number on her business card and email .sig, she just never answered or checked voicemail. When she took a new job another colleague inherited her phone number and when he went to set up his voicemail there were basically eight years’ worth of messages left for her that he had to delete.
The autoresponder messages give information like how long the person you are reaching is unavailable and who else is the person in the absence and from when will the person be back available. Make sure the message is not too lengthy and is set up easily and conveniently understood by the email users. Try to manage your absence in such a way that the client doesn’t get affected and uninformed.

A word of warning: These greetings will not do you any favors if you're in the midst of a job hunt or work in a conservative industry. Always remember your target personas. If there's a chance they won't appreciate your sense of humor, opt for a straightforward greeting instead.
An out of office message could become an opportunity to connect with your client on a more personal level. Consider sharing something about yourself that they might not have known about while working with you.