I rang back the number 0448 365 421, spoke with someone who hung up right away

Answered call and nobody some back. Total silence on the other end, so I hung up. 10 minutes later I get a very convincing scam SMS saying that a package for me has outstanding shipping fees.

Australia Post warning about shipping scams. https://auspost.com.au/about-us/about-our-site/online-security-scams-fraud/scam-alerts

When you answer 0448 365 421 they hang up and then when you call back the phone rings out

They said hello to which I replied and then they said hello again to which I replied again followed by silence so I just hung up

Called back number and get automated message 'the number you have called is not in service'

I also had a phone call from this number and the a SMS saying that a package for me has outstanding shipping fees.

I had the same. Message about outstanding shipping fee and this number rang.

The same number 0448 365 421 appeared as a missed call on my ph and wanted to add on my contact as an emergency contact number.

Tried to call back - line busy - probably a scam

When picked up the phone, no respond on the other side. I don’t speak and hang up straight away as it could be scam call and recording my voice for hacking or illegal purposes.

Call dropped when I answered, when I called back the other party just hangs up.