Tried calling back & immediately got recorded message, "Optus advises that the mobile number you have dialled has been disconnected."
By the way my phone strongly suggested it was SPAM.

Missed their first call. Called back but got a robot voice saying the number was disconnected.

Second call came through a few hours later. I answered but just heard background noise (like a call center or busy office) and they hung up after about 20 seconds without saying a word.

This number called, no voicemail left. I think most if not all 9099 are spoofed. There should be a national block on these numbers with an opt-in for those who want to use it legitimately.