“Good afternoon. You have reached the office of [your name]. I will be out of the office beginning on [date] and will be returning on [date]. Please leave a brief message with your contact information, and I’ll be sure to get back to you as soon as I return on [date]. If this is an emergency or you need to speak with someone before I return, please contact [name of colleague/supervisor], [their job title], at [their phone number].”
“If I leave a voicemail message on my first call in the morning, and I get voicemail again on my second attempt (either later that day or the next time I try and reach them) what do I say when that happens?”

Never use voicemail as a way to avoid speaking with someone. It doesn't help, and a person would have to be terminally dense not to figure out your game. If you must call when you know the other person isn't available, say, "I know you won't be able to take the call now, but I wanted to let you know that..."
State your number right after your name. Many people wait until the very end of the message to state their number. This will irritate the receiver of your message because if he doesn’t get it down, he then has to sit through the whole damn message again to hear it repeated.