"Hi! John's answering machine is broken. This is his refrigerator. Please speak very slowly, and I'll stick your message to myself with one of these magnets."
You can operate your answering system from a remote location using any touch-tone telephone or the handset if it allows that capability. You can check, play, or delete messages, even record a new greeting message when you are away from home. Additionally, you can turn On or Off your answering system from a remote location depending on model.

You might also consider giving them the option to skip your voicemail message completely by pressing another key (again, check with IT). You also want to make callers aware what you will return their call, instead of telling them that you will try to return their call--and let them know when you are likely to do it, if possible. If necessary, provide additional or emergency information in your voicemail greetings. My dentist, for example, lists his cell phone number for emergencies.
Sound benign and businesslike? That's the whole idea. As Karyn-Ruth explains it, "Bill collectors, the IRS, sales personnel, obscene callers and other obnoxious people think they have reached a business, and therefore the wrong number, and hang up."