Mine is so simple..."Hi, this is (phone number). Leave a message after the beep."
I learned after living the dorms that it's best to have a message where you don't identify yourself directly...you get funnier messages that way because people think they've called who they're supposed to call
Once I got a message from someone's grandma..."Miles, your grandpa and I are wondering what your address is, we have a package to send you...your mother said you needed socks so I bought you some socks..."