Never ever dial back unless you recognize the number. Here is a scam that is popular right now:
Someone receives a call, typically on a cell phone, from an international phone number that hangs up and leaves a “missed call” but no voicemail. If the receiver calls the number back, there will be an international connection fee plus a per minute rate. Some of these fees are $20 per call, plus $9 per minute. If you receive a call from a number that you do not recognize, do not call it back. The cellular carriers have advised us they are not waiving these fees as they view them as calls made by the customer.