Anytime an unknown caller leaves a voice mail that is "urgent" -- and they request that you "call us back immediately." Or any words that relate to an "emergency, Immediately." It's most certainly fraud.
Especially when a Google search of the number reveal No Company Name. Be careful of this too, because lots of crooks are spoofing reputable companies phone numbers.
Best solution... Don't answer the phone for ANY UNKNOWN CALLERS. if it's important, it can wait until you verify the phone number.