Keep getting calls from this number (unidentified caller) who does not leave a message.  Will block this caller.  Adding call to list for further identification by others.

We just received a call from this number and I answered. After saying "hello" twice and assuming it was an auto-dialer that had called, there was a loud beep and then a recorded voice said "Goodbye." But no idea who it was that called! When I called the number back, I got "The number you dialed is not in service." But I'm guessing this is a dummy recording because it doesn't sound the same as others I usually get when that's the case.