Calling several times. No message and I can't find the number listed anywhere as a registered legit phone number (or is being spoofed)

This call showed as coming from Sydney, NSW. I had missed the call and the caller reached my message bank, but left no message. When I called the number back, it gave a continually engaged signal, which indicates to me that it was probably a computerized scammer, trolling its way through a large list of telephone numbers.