Same here but I answered and they just didn't say anything. I thought it was a person on the other end though. Ya know how sometimes the background noise is a little different, like the air itself has sound but with a machine there isn't any? hmm... I also received one from 44-207-403-5019, same situation, five days before.
I looked up the country's extension and it was for the United Kingdom. (All the epsiodes of Carmen Sandiego are coming back to me now, haha.)