Mine is "Hello, you've reached the voicemail of the IT Department, please leave a message and we'll get back to you".
Honestly, if they're so stupid that they need instructions on how to leave a message (like telling them to leave a number), then I really don't want to talk to them. Then again, my voicemail system records the number they're calling from anyway, so it's generally not required.
Not that I ever phone anyone back anyway. I'm an e-mail only kind of guy.