It's an office, so you don't want your message to be too lively! How about:
'Hello, this is Ju (I would give first and last names, unless the office is extremely informal), secretary (or, 'assistant') to Mr. Leong. If you are calling during normal business hours, I am away from my desk or on another line and unable to take your call. If you leave your name, number and a brief message, I will be sure to return your call as soon as possible. Thanks for calling. Have a great day!
Then: be sure you do return the calls as soon as possible. It is highly annoying to hear this message again without having received a call back.
I hope this is helpful,
- For Teachers