They left no message, and ate always busy when I try to call back. (TIP: before you call back any unknown number, change your call info to "private" by putting *67 before the phone number you're dialing)
Tip: that doesn't always work. You can't hide your caller ID from an 800 number and certain other types of phone systems since the information is transmitted via ANI.
If it's an unknown number, why would you call them back? If they can't bother to identify themselves or leave a detailed message, the call obviously isn't important and not worth your time calling back.
When i called back i got a automated machine that was for a music station, it was talking about some music concert and how if you won the little trivia game they had you could win 2 free tickets. Then i hung up