<TL;DR> I initiated the call to a legitimate Optus billing number and asked them to call me back about another issue.
They called me back from this number. It is a legitimate Optus number. Unless spoofed. Do not tap to call back. Use keypad to manually enter number if calling back.
But because I am a victim of the Breech, as I do with any company now, if this number had contacted me 1st, without me asking them to do so, I would have asked for a contact name and department and said that I
It looks legit because of a follow up that went well but what I think might be happening is that the same number is being hijacked by the hackers and misused. Optus need to change their official one