how to record a second voicemail greeting?

It seems so logical that you could record multiple voicemail greetings, that I must be missing something. How do I record a second or third or fourth voicemail greeting so that I can easily switch greetings when needed? I get tired of re-recording my voicemail greeting when I'm away from the office for a day - I'd like to record a greeting once to say that I'm not available today vs. my standard greeting that says I'll get back to you ASAP.