I called back number (02) 8015 6203 rang 3 times then goes to busy tone.

This number has called my mobile 3 times this week since 31 May 2017. Each time I answered the call no one speaks but it could be a remote calling device because when I called the number back, I got a pre-recorded message saying that "Extension 4001 is unavailable. Please leave a message........" in an American accent. Sounds like a scam of some sort.