If you did not know the person or number and were not expecting a call from that location, why did you call back?  Unless you have friends or relatives who hate answering machines and won't leave a message, anyone you actually wished to talk to would leave a message.

Rule #1 Do not answer calls from unfamiliar names or numbers.

Rule #2 Do not call back unknown numbers that do not leave a message with a valid reason to call them back.