Pretended to be NBN, talked about someone having logged into our line, etc. Then started asking questions, pretending to want to secure the line. Basically was getting to the point where he was going to ask security questions and so on, I hung up. But if NBN detects unsolicited activity they don't need to call you to sort this out or to secure your line, they just do that...that's their job. They also don't call from a mobile phone line! Hence clearly a scam!