Never call them back! If they are a legitimate company, they will leave a message. You have nothing to gain by calling back. All you are doing is confirming they reached a valid number. Or, they may have spoofed the number and you could be calling an innocent party.
The call came to my office phone, which is a non-listed Internet-based number. When I answered, there was no sound on the other end of the line, then the call was disconnected by the caller.