Crank calls?

God knows that I feel so sleepy now :( I was awake by crank calls from unknown SGP numbers (I noticed that the first 5 numbers are the same) for two consecutive nights at around the same time i.e. 3:3X a.m.! I managed to answer one of the call today, with an unconscious mind i.e. half asleep & half awake condition. It was a male voice - speaking in English on the other end. Before I can barely figure out what he is saying, I told him that he had dialled the wrong number & I hang up!

Since I was wide awake then, I called the specific number i.e. the number I had received the call from with the purpose of finding out who that person is and what is the person's purpose of calling. However, no one pick up after a few times of trying. I wonder ...