The BBC website has an article on “number spoofing” being used in phone based scams to get private info.
This could well be happening in NZ already! I frequently have my home line forwarded to my mobile and there have been numerous times when I’ve tried to return a missed call and the number doesn’t exist. Or when one of my kids has taken a call and I’ve tried to call back because the message they’ve taken hasn’t made sense.
When I was a kid I was taught that if you are unsure about anyone’s identity find out who they are and then look up their phone number, call and verify if they are meant to contacting you. The context we were given was fake policemen, but the rule works for any kind of faker!
If you take a call and are dubious ask for their extension but find the phone number yourself and call them back.