Open your voicemail greeting with a 'Thank you for your call. We're not available right now by please leave your message and we'll get back to you within [specify time]'.
There are two ways to add a greeting for the Auto Attendant. You can upload an already recorded sound file or record it on the go by calling the Auto Attendant's number or extension and pressing *5 followed by PIN when prompted from any device or cellphone.

“Hi, you’ve reached [your name]. I’m away from[date] to [date]. If you need help with [X] before then, please contact [name] at [phone number]. Everyone else, please leave your name and number and I’ll return your call when I return. Thanks and have a great day.” “Hello, you’ve reached [your name]. I’m currently [exploring Asia, hiking through the jungle in Costa Rica, hanging out on the beach in Bermuda] — or more likely, [recovering from extreme jet lag, googling ‘Are red spiders poisonous,’ or looking for SPF 150 sunscreen] and won’t be back in the office until [date]. Leave your contact info and reason for calling and I’ll get in touch then.” “Hey there, this is [your name] from [your company]. I’m out of the office until [date]. In the meantime, please direct your inquiries to [coworker’s name] at [email address]. [He, she] can also be reached at [phone number]. Thank you.”
Keep your personal voicemail greeting as short and to the point as possible, even if you are injecting a little creativity into it. This is especially important if you expect to receive a lot of long-distance phone calls or calls from those on different cell phone networks, as many phone companies charge by the minute. Think about your personal preferences when you reach someone’s voicemail, and tailor your own greeting towards that.